Add x86_64-unknown-linux-musl build support

Author: martell

.travis.yml

@@ -31,6 +31,7 @@ matrix:
     - { <<: *linux, env: SKIP_TESTS=1 TARGET=mips64-unknown-linux-gnuabi64   }
     - { <<: *linux, env: SKIP_TESTS=1 TARGET=mips64el-unknown-linux-gnuabi64 }
     - { <<: *linux, env: SKIP_TESTS=1 TARGET=s390x-unknown-linux-gnu         }
+    - { <<: *linux, env: SKIP_TESTS=1 TARGET=x86_64-unknown-linux-musl       }
     - { <<: *linux, env: SKIP_TESTS=1 TARGET=arm-linux-androideabi           }
     - { <<: *linux, env: SKIP_TESTS=1 TARGET=armv7-linux-androideabi         }
     - { <<: *linux, env: SKIP_TESTS=1 TARGET=aarch64-linux-android           }

README.md

@@ -659,6 +659,7 @@ platform of your choice:
 - [x86_64-pc-windows-msvc](https://static.rust-lang.org/rustup/dist/x86_64-pc-windows-msvc/rustup-init.exe)<sup>[†](#vs2015)</sup>
 - [x86_64-unknown-freebsd](https://static.rust-lang.org/rustup/dist/x86_64-unknown-freebsd/rustup-init)
 - [x86_64-unknown-linux-gnu](https://static.rust-lang.org/rustup/dist/x86_64-unknown-linux-gnu/rustup-init)
+- [x86_64-unknown-linux-musl](https://static.rust-lang.org/rustup/dist/x86_64-unknown-linux-musl/rustup-init)
 - [x86_64-unknown-netbsd](https://static.rust-lang.org/rustup/dist/x86_64-unknown-netbsd/rustup-init)
 
 <a name="vs2015">†</a>

ci/docker/x86_64-unknown-linux-musl/Dockerfile

@@ -0,0 +1,11 @@
+FROM alpine:3.9
+
+RUN apk update && \
+  apk add \
+  curl \
+  ca-certificates \
+  perl \
+  make \
+  gcc
+
+ENV CC_x86_64_unknown_linux_musl=gcc