Make the check for a struct fitting in a register check for either managed or native...

Make the check for a struct fitting in a register check for either managed or native depending on the situation (i.e. managed call, reflection, P/Invoke) instead of only for managed. Fixes dotnet/coreclr#20702.

Clean up duplicate #ifdefs.

Move #ifdef into method impl instead of outside the implementations.

Move IsRegPassedStruct out of UNIX_AMD64_ABI #ifdef.

Move check for enregistered struct out of UNIX_AMD64_ABI #ifdef

Add dummy implementation of IsRegPassedStruct and IsNativeStructPassedInRegisters when UNIX_AMD64_ABI isn't defined.

Commit migrated from https://github.com/dotnet/coreclr/commit/014a56de050a8c36f1c4ae38a84891a09d17469f