Remove the automatic ISA assumption in the cranelift-codegen build.rs

[lambdadeltakay]

Feature

Allow for specifiying no ISA targets and not failing to compile in the build.rs when no ISA targets are specified on a unsupported platform for code generation on cranelift-codegen

Benefit

This would allow for using cranelift related utilities on platforms like wasm or powerpc.

Implementation

This would require just removing the code in the build.rs that assumes the ISA target for generation when no ISA targets are specified

Alternatives

A possible alternative would be a feature flag that would specify that no ISA targets should be built in

[bjorn3]

I believe disabling the host-arch feature of cranelift-codegen will already do this.

[lambdadeltakay]

wasmtime/cranelift/codegen/build.rs

Lines 42 to 51 in 1fa8de1

	// Don't require host isa if under 'all-arch' feature.
	let host_isa = env::var("CARGO_FEATURE_HOST_ARCH").is_ok() && !all_arch;
	if isas.is_empty() || host_isa {
	// Try to match native target.
	let target_name = target_triple.split('-').next().unwrap();
	let isa = meta::isa_from_arch(&target_name).expect("error when identifying target");
	println!("cargo:rustc-cfg=feature=\"{}\"", isa);
	isas.push(isa);
	}

From my experiments and these lines of code it doesn't seem to do this, instead I have to manually set at least one ISA that doesn't relate to the target to avoid a compile error