[compiler-rt] Add extendhfxf2 into compiler rt (PR #113897)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

This also broke the compiler-rt tests on mingw, again: https://github.com/mstorsjo/llvm-mingw/actions/runs/12021713343/job/33522994914 and https://github.com/mstorsjo/llvm-mingw/actions/runs/12021713343/job/33522995313

I can test the incremental fix in #117665.

https://github.com/llvm/llvm-project/pull/113897