Support riscv64gc-unknown-linux-musl (alexcrichton#202)

Author: vi

src/lib.rs

@@ -299,6 +299,7 @@ impl Build {
             "powerpc64le-unknown-linux-musl" => "linux-ppc64le",
             "riscv64gc-unknown-freebsd" => "BSD-riscv64",
             "riscv64gc-unknown-linux-gnu" => "linux-generic64",
+            "riscv64gc-unknown-linux-musl" => "linux-generic64",
             "riscv64gc-unknown-netbsd" => "BSD-generic64",
             "s390x-unknown-linux-gnu" => "linux64-s390x",
             "sparc64-unknown-netbsd" => "BSD-generic64",