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

This reverts commit c64ca930.

This patch tripped a few build bots:

  http://lab.llvm.org:8011/builders/clang-x86_64-debian-fast/builds/24703/
  http://lab.llvm.org:8011/builders/clang-cmake-x86_64-avx2-linux/builds/13465/
  http://lab.llvm.org:8011/builders/clang-with-lto-ubuntu/builds/15994/

Reverting to clear the bots.