Fix incorrect thread affinitization (dotnet/coreclr#24045)

Fix incorrect thread affinitization (dotnet/coreclr#24045)

The PAL_SetCurrentThreadAffinity was incorrectly adding the specified processor
to the current thread affinity set instead of setting the affinity to only
the processor specified.
It was causing significant performance hit in aspnet benchmarks on machines with
many cores.
This change crept in when I was refactoring the related code while removing
CPU groups emulation.

Commit migrated from https://github.com/dotnet/coreclr/commit/2a6ca5c3eb8f5597be776aea85fb4086a53ac0ad