[builtins] Build with -Wbuiltin-declaration-mismatch if supported

GCC is able to check that the signatures of the builtins are as expected
and this shows some incorrect signatures on ld80 platforms (i.e. x86).
The *tf* functions should take 128-bit arguments but until the latest fixes
they used 80-bit long double.

Differential Revision: https://reviews.llvm.org/D153814