[msan] Check for AVX regs using offset

glibc versions < 2.26 use different names for the fields.
However the layout is unchanged, so using the offset should be a
portable way to address this issue across platforms.

Fixes: https://github.com/llvm/llvm-project/issues/53014

Patch By: paulkirth

Differential Revision: https://reviews.llvm.org/D116695