llvm6: Remove MIPS64 archive variant

It looks like LLVM also removed it in llvm-mirror/llvm@f45adc29d in favor of the
name "GNU64". This was added in the thought that we'd need such a variant when
adding mips64 support but we ended up not needing it! For now let's just
removing the various support on the Rust side of things.

src/librustc_llvm/ffi.rs

@@ -330,7 +330,6 @@ pub enum DiagnosticKind {
 pub enum ArchiveKind {
     Other,
     K_GNU,
-    K_MIPS64,
     K_BSD,
     K_COFF,
 }

src/librustc_llvm/lib.rs

@@ -105,7 +105,6 @@ impl FromStr for ArchiveKind {
     fn from_str(s: &str) -> Result<Self, Self::Err> {
         match s {
             "gnu" => Ok(ArchiveKind::K_GNU),
-            "mips64" => Ok(ArchiveKind::K_MIPS64),
             "bsd" => Ok(ArchiveKind::K_BSD),
             "coff" => Ok(ArchiveKind::K_COFF),
             _ => Err(()),

src/rustllvm/ArchiveWrapper.cpp

@@ -42,7 +42,6 @@ struct RustArchiveIterator {
 enum class LLVMRustArchiveKind {
   Other,
   GNU,
-  MIPS64,
   BSD,
   COFF,
 };
@@ -51,8 +50,6 @@ static Archive::Kind fromRust(LLVMRustArchiveKind Kind) {
   switch (Kind) {
   case LLVMRustArchiveKind::GNU:
     return Archive::K_GNU;
-  case LLVMRustArchiveKind::MIPS64:
-    return Archive::K_MIPS64;
   case LLVMRustArchiveKind::BSD:
     return Archive::K_BSD;
   case LLVMRustArchiveKind::COFF: