Reapply "clang: Treat ieee mode as the default for denormal-fp-math"

This reverts commit 737394c4.

The fp-model test was failing on platforms that enable denormal flushing
based on -ffast-math. This needs to reset to IEEE, not the default in
these cases.

Change-Id: Ibbad32f66d0d0b89b9c1173a3a96fb1a570ddd89