rustc_trans: clobber $1 (aka $at) on mips

This copies what clang does. There is a long explanation as to why this
is needed in the clang source (tools/clang/lib/Basic/Targets/Mips.h).

src/librustc_trans/asm.rs

@@ -60,6 +60,7 @@ pub fn trans_inline_asm<'a, 'tcx>(
     // Basically what clang does
     let arch_clobbers = match &bx.sess().target.target.arch[..] {
         "x86" | "x86_64"  => vec!["~{dirflag}", "~{fpsr}", "~{flags}"],
+        "mips" | "mips64" => vec!["~{$1}"],
         _                 => Vec::new()
     };