[flang] Remove fp128 support for llvm.round and llvm.trunc

The fp128 in llvm.round and llvm.trunc is not supported in X86_64 for
now. Revert the support. To support quad precision for llvm.round and
llvm.trunc, it may should be supported using runtime.

Reviewed By: Jean Perier

Differential Revision: https://reviews.llvm.org/D130556