[X86] Change legalization action for f128 fadd/fsub/fmul/fdiv from Custom to LibCall.

[X86] Change legalization action for f128 fadd/fsub/fmul/fdiv from Custom to LibCall.

The custom code just emits a libcall, but we can do the same
with generic code. The only difference is that the generic code
can form tail calls where the custom code couldn't. This is
responsible for the test changes.

This avoids needing to modify the Custom handling for strict fp.