Update ICorDebugInfo::RegNum -> CONTEXT offset mapping (GetRegOffsInCONTEXT) for XMM registers

[tommcdon]

Tracking issue for a review comment on #129992.

The AMD64 RegNum enum now includes XMM0–XMM15. Any runtime code that maps ICorDebugInfo::RegNum to CONTEXT offsets (e.g. GetRegOffsInCONTEXT in vm/util.cpp) must be updated accordingly; today the TARGET_AMD64 switch only handles GPRs and will treat the new XMM RegNum values as invalid. That can break native var access for VLT_REG_FP and the new mixed int+fp VLT_REG_REG cases on AMD64.

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