Add tests for default value of Tag_ABI_FP_rounding.

Change-Id: I051866d073fc6ce87ce3e693a3762da6d81f4393

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223217 91177308-0d34-0410-b5e6-96231b3b80d8