[llvm] ARM: Remove redundant or buggy config of __aeabi_d2h (PR #152126)

[Eli Friedman via llvm-commits]

https://github.com/efriedma-quic approved this pull request.

See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120897 for a summary of the history here; it's very messy.  Really, we should be calling `__gnu_d2h_ieee` on targets using libgcc because `__aeabi_d2h` doesn't exist, but the patch (https://reviews.llvm.org/D94557) never got merged.

I think we have sufficient test coverage in llvm/test/CodeGen/ARM/fp16.ll, to detect if anyone accidentally makes further changes here.

This seems fine for now; if someone wants to revive D94557, they can do it once all the TableGen changes are in.

LGTM

https://github.com/llvm/llvm-project/pull/152126