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

69660ccf broke the [Solaris/sparcv9
buildbot](https://lab.llvm.org/staging/#/builders/12/builds/264):
`compiler-rt/lib/builtins/int_to_fp.h` unconditionally uses `*int128_t`
which don't exist on 32-bit SPARC.

As suggested in https://github.com/llvm/llvm-project/pull/67540, this
patch fixes this by moving the `CRT_HAS_TF_MODE` guard up which does the
necessary checks.

Tested on `sparcv9-sun-solaris2.11`.