LTO build: fix perf regression from strict alias violation and incorrect inline assem...

LTO build: fix perf regression from strict alias violation and incorrect inline assembly (dotnet/coreclr#7018)

Fix inline assembly implementation of getextcpuid() and strict aliasing violations in its callers

The implementation writes to the unsigned char result array, so
the clobber "memory" is needed.

As well its callers have strict aliasing violations from reading the
result array as a DWORD array. The fix is to memcpy the resulting
buffer onto the DWORD array.

Commit migrated from https://github.com/dotnet/coreclr/commit/882a634da1fb2c6acc0b2414be43ceed28544fbf