Auto merge of #143412 - Kobzol:std-detect-in-stdlib, r=Amanieu

Move `std_detect` into stdlib

This PR moves the `std_detect` crate from `stdarch` to be a part of rust-lang/rust instead.

The first commit actually moves the whole directory from the stdarch Josh subtree, so that git blame history is kept intact. Then I had to make a few changes to appease `tidy`.

The most complex thing here is porting the tests. We can't have `std_detect` both in r-l/r and stdarch, because they could get desynchronized, so we have to perform the move more or less "atomically", which means that we also have to port all the existing `std_detect` tests from the `stdarch` repository.

The stdarch repo runs the following `std_detect` tests:

### Build
The `build-std-detect.sh` script (https://github.com/rust-lang/stdarch/blob/e2b6512aed87df45294ae680181eeef7a802cd95/ci/build-std-detect.sh) builds `std_detect` using the nightly compiler for several targets. This will be subsumed by normal `x build library` on our Tier 1/2 targets. However, the stdarch repository also tests the following targets:
- aarch64-unknown-freebsd
- armv6-unknown-freebsd
- powerpc-unknown-freebsd
- powerpc64-unknown-freebsd
- aarch64-unknown-openbsd

Which we don't build/test on our CI currently. I think we have mostly two options here:
1) Ignore these targets
2) Create a special CI job that will build stage 1 rustc and then cross-compile std (or just the `std_detect` crate?) for these targets.

### Documentation
The `dox.sh` script (https://github.com/rust-lang/stdarch/blob/3fec5adcd52a815f227805d4959a25b6402c7fd5/ci/dox.sh) builds and documents `std_detect` for several targets. All of them are Tier 2/we have `dist-` jobs for them, so I think that we can just skip this and let our normal CI subsume it?

### Tests
The `run.sh` script (https://github.com/rust-lang/stdarch/blob/1b201cec2cca7465602a65ed6ae60517224b15f3/ci/run.sh) runs `cargo test` on `std_detect` with a bunch of variations of feature flags. This will be subsumed by `x test library` in our CI. The only problem is that `stdarch` runs these tests for a ludicrous number of targets:
```
        - tuple: i686-unknown-linux-gnu
        - tuple: x86_64-unknown-linux-gnu
        - tuple: arm-unknown-linux-gnueabihf
        - tuple: armv7-unknown-linux-gnueabihf
        - tuple: aarch64-unknown-linux-gnu
        - tuple: aarch64_be-unknown-linux-gnu
        - tuple: riscv32gc-unknown-linux-gnu
        - tuple: riscv64gc-unknown-linux-gnu
        - tuple: powerpc-unknown-linux-gnu
        - tuple: powerpc64-unknown-linux-gnu
        - tuple: powerpc64le-unknown-linux-gnu
        - tuple: s390x-unknown-linux-gnu
        - tuple: i586-unknown-linux-gnu
        - tuple: nvptx64-nvidia-cuda
        - tuple: thumbv6m-none-eabi
        - tuple: thumbv7m-none-eabi
        - tuple: thumbv7em-none-eabi
        - tuple: thumbv7em-none-eabihf
        - tuple: loongarch64-unknown-linux-gnu
        - tuple: wasm32-wasip1
        - tuple: x86_64-apple-darwin
        - tuple: x86_64-apple-ios-macabi
        - tuple: aarch64-apple-darwin
        - tuple: aarch64-apple-ios-macabi
        - tuple: x86_64-pc-windows-msvc
        - tuple: i686-pc-windows-msvc
        - tuple: aarch64-pc-windows-msvc
        - tuple: x86_64-pc-windows-gnu
        - tuple: aarch64-unknown-linux-gnu
        - tuple: aarch64_be-unknown-linux-gnu
        - tuple: armv7-unknown-linux-gnueabihf
        - tuple: loongarch64-unknown-linux-gnu
        - tuple: powerpc-unknown-linux-gnu
        - tuple: powerpc64-unknown-linux-gnu
        - tuple: powerpc64le-unknown-linux-gnu
        - tuple: riscv32gc-unknown-linux-gnu
        - tuple: riscv64gc-unknown-linux-gnu
        - tuple: s390x-unknown-linux-gnu
        - tuple: x86_64-unknown-linux-gnu
        - tuple: aarch64-apple-darwin
        - tuple: aarch64-apple-ios-macabi
```
We definitely do not run *tests* for all of these targets on our CI.

# Outcome
We have decided to just subsume std_detect tests by our normal test suite for now, and not create a separate CI job. Therefore, this PR performs the following changes in target testing for `std_detect`:

The following T3 targets would go from "build" to "nothing":
```
aarch64-unknown-freebsd (T3)
armv6-unknown-freebsd (T3)
powerpc-unknown-freebsd (T3)
powerpc64-unknown-freebsd (T3)
aarch64-unknown-openbsd (T3)
```

The following T3 targets would go from "test" to "nothing":
```
aarch64_be-unknown-linux-gnu (T3)
riscv32gc-unknown-liux-gnu (T3)
```

The following T2 targets would go from "test" to "build":
```
arm-unknown-linux-gnueabihf (T2)
armv7-unknown-linux-gnueabihf (T2)
riscv64gc-unknown-linux-gnu (T2)
powerpc-unknown-linux-gnu (T2)
powerpc64-unknown-linux-gnu (T2)
powerpc64le-unknown-linux-gnu (T2)
s390x-unknown-linux-gnu (T2)
i586-unknown-linux-gnu (T2)
loongarch64-unknown-linux-gnu (T2)
wasm32-wasip1 (T2)
x86_64-apple-ios-macabi (T2)
aarch64-apple-ios-macabi (T2)
aarch64-pc-windows-msvc (T2)
armv7-unknown-linux-gnueabihf (T2)
loongarch64-unknown-linux-gnu (T2)
powerpc-unknown-linux-gnu (T2)
```

I have confirmed in rust-lang/stdarch#1873 that the current version of this PR would pass stdarch's CI testsuite.

r? `@ghost`

try-job: armhf-gnu
try-job: arm-android

Author: bors