Unix/x64 ABI cleanup

Unix/x64 ABI cleanup

Eliminate `FEATURE_UNIX_AMD64_STRUCT_PASSING` and replace it with `UNIX_AMD64_ABI` when used alone. Both are currently defined; it is highly unlikely the latter will work alone; and it significantly clutters up the code, especially the JIT.

Also, fix the altjit support (now `UNIX_AMD64_ABI_ITF`) to *not* call `ClassifyEightBytes` if the struct is too large. Otherwise it asserts.