[builtins] Hide long double fp_test.h helpers if it's not fp128.

Like we do for the various __*tf* tests, check that long double is the
128bit type we expect directly in the header. The latter is now used by
unrelated tests (__*hf* since r237161), and those tests will break for
no reason if uint128_t doesn't exist, and long double isn't fp128.

llvm-svn: 239630