Adding support for X86Base.CpuId (#40167)

Adding support for X86Base.CpuId (#40167)

* Adding support for X86Base.CpuId

* Rename getcpuid and getextcpuid to __cpuid and __cpuidex, respectively

* Removing xchg from the Unix x64 __cpuid implementation

* Add a comment as to why the X86/X86Base/CpuId test limits the checked vendors

* Apply suggestions from code review

Co-authored-by: Jan Kotas <jkotas@microsoft.com>
* Adding back a missing parentheses

* Fixing a typo in the isGenuineIntel check

* Avoid a conflict around cpuInfo

* Avoid an implicit cast when comparing the cpuidInfo

* Separate the __cpuidex qcall into coreclr and mono specific variants

* Add the partial modifier to the X86Base.PlatformNotSupported.cs file

Co-authored-by: Jan Kotas <jkotas@microsoft.com>