Fix llvm register allocator for native/cross build differences

Work around an issue in LLVM's register allocator, which can cause
slightly different i386 object files, when produced by a native or cross
build of clang.

This adds another volatile qualifier to a float variable declaration in
the weightCalcHelper() function, which otherwise produces slightly
different float results on amd64 and i386 hosts. In turn, this can lead
to different (but equivalent) register choices, and thus non-identical
assembly code.

See https://github.com/llvm/llvm-project/issues/99396 for more details.

Note this is a temporary fix, meant to merge in time for 13.4. As soon
as upstream has a permanent solution we will import that.

PR: 276961
Reported by: cperciva
MFC after: 3 days