gecko: unbreak aarch64 build for non-empty CPUTYPE
As of Rust 1.22.1 the following CPUTYPEs are supported:
https://github.com/rust-lang/llvm/blob/c7a16bd57c2a/include/llvm/Support/AArch64TargetParser.def#L54-L77
gecko: unbreak aarch64 build for non-empty CPUTYPE Tags None Referenced Files None Subscribers None 
Description 
Details 
  |