compiler_flags: Change optimization_fast from '-Ofast' to '-O3'

[thughes]

'-Ofast' enables optimizations that are not standards compliant, which can
produce unexpected results unless used carefully.

When building with clang 19 and newer, it warns:

clang: error: argument '-Ofast' is deprecated; use '-O3 -ffast-math' for
the same behavior, or '-O3' to enable only conforming optimizations
[-Werror,-Wdeprecated-ofast]

Relevant discussion:

• [clang] Add deprecation warning for -Ofast driver option llvm/llvm-project#98736
• https://discourse.llvm.org/t/rfc-deprecate-ofast/78687/109
• https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115990

[thughes]

There are other possibilities here, such as disable the warning for clang, only change clang to use -O3, or change clang to use -O3 -ffast-math to be similar to gcc.

However, it seems like we want the meaning of this flag to be the same across all compilers, so I think it makes sense for whatever we do to be consistent. If we want to keep the unsafe optimizations, it might be good to rename it to something like optimization_fast_unsafe so people pay attention when using it.

[thughes]

If we choose to make this change, I'll add a context comment here so it doesn't accidentally get changed back in the future.

[nordicjm]

PR says changing for clang but is changing for gcc, clang changes should go in the clang file as per https://github.com/zephyrproject-rtos/zephyr/blob/a52b9d4084133edbdf547f308f5cb349a764fcc3/cmake/compiler/clang/compiler_flags.cmake#L1-L4]

GCC has no plans to deprecate this flag https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115990

[thughes]

I think we want to be consistent on this on all compilers.

-Ofast is not standards compliant on both gcc and clang. We probably don't want people to use it unless they're really sure that it's safe for the code they're enabling it on. The name optimization_fast suggests that it's just faster, not necessarily unsafe. We could rename it to optimization_fast_unsafe to make it clear that it's not compliant. Or we could just choose to make optimization_fast be something that is compliant like I proposed in this change. (It's not clear whether the unsafe aspect was taken into account when it was enabled for the code that is currently using it.)