[compiler-rt] [builtins] Fix floattitf.c etc. compilation on Solaris/SPARC (PR #70058)

[Rainer Orth via llvm-commits]

rorth wrote:

> LGTM, alternatively could also add an explicit check for INT128 availability to this `#if` rather than relying on CRT_HAS_TF_MODE only being defined when int128 also exists.

I've committed the patch as is to quickly unbreak the buildbot.  If we want to add more checks to make headers self-contained, I guess it's better to do this consistently rather than just in a single place.

https://github.com/llvm/llvm-project/pull/70058