NetBSD 6.99.26 switched to default rounding mode, so adjust
__FLT_EVAL_METHOD__ accordingly. Add test case for this and the SSE2
variances on NetBSD.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194377 91177308-0d34-0410-b5e6-96231b3b80d8