[compiler-rt] [builtins] Support building the 128-bit float functions on ld80 platforms (PR #68132)

[Alexander Richardson via llvm-commits]

arichardson wrote:

@perry-ca Could you open this as a pull request? The current diff is quite hard to read. I'm not sure about the changes to compiler-rt/lib/builtins/fp_lib.h but otherwise I think it looks sensible.

https://github.com/llvm/llvm-project/pull/68132