Enable optimized single-proc allocation helpers for single-proc x86/x64 systems only...

Enable optimized single-proc allocation helpers for single-proc x86/x64 systems only (dotnet/coreclr#27014)

Use maximum number of processors the process may run on to determine whether it is ok to use
single-proc allocation helpers. It is not sufficient to depend on current process affinity since
that can change during the process lifetime.

Also, the single-proc allocation helpers work well on x86/x64 systems only because of they depend
on atomic non-interlocked increment instruction for good performance. Such instruction is available
on x86/x64 only. Disable them everywhere else.

Fixes dotnet/coreclr#26990

Commit migrated from https://github.com/dotnet/coreclr/commit/4e702da2c06ded8239d5eb2f47619604efbf3fde