build fails on arm64

[vielmetti]

Building on a Packet c1.large.arm (arm64, ThunderX) system I get this error:

error[E0425]: cannot find function `parse_x86_cpuid` in this scope
  --> cranelift-native/src/lib.rs:45:9
   |
45 |         parse_x86_cpuid(&mut isa_builder)?;
   |         ^^^^^^^^^^^^^^^ not found in this scope
error: Could not compile `cranelift-native`.

is there arm64 support in this package? I did not expect to get this codepath.

[bnjbvr]

What happens is that the method exists if and only if we're on the expected target, but the way cfg!(...) is replaced by the boolean value at compile time, meaning that the resulting code is e.g. if true { parse_x86_cpuid(...) } (or on your platform, it'd be if false {).

I think we need to stub out the function in the other case, or use an additional dependency cfg-if to properly do this.