runtime: Add architectures supported by Mono but not supported by CoreCLR to System.Runtime.InteropServices.Architecture

Mono consumes this enum, but also supports architectures that aren’t 32/64-bit x86/ARM; in particular, PowerPC and WebAssembly. (SPARC and MIPS are supported, but being removed for 6.0 unless someone does something otherwise.)

To bikeshed on possible values:

• PowerPC (32-bit PowerPC)

• PowerPC64 (64-bit PowerPC)

• S390X, related issue: https://github.com/dotnet/runtime/issues/34195

* WebAssembly (was added)

OSPlatform could also use other OSes, but string construction of new ones works in the meantime as CoreFX doesn’t want to add API surface until CoreCLR supports the OS.

Propsoal

namespace System.Runtime.InteropServices
{
    public enum Architecture
    {
        X86,
        X64,
        Arm,
        Arm64,
+       PowerPC,
+       PowerPC64,
+       S390X,
    }
}