Add missing FP build attribute tests.

The test file test/CodeGen/ARM/build-attributes.ll was missing several
floating-point build attribute tests. The intention of this commit is that for
each CPU / architecture currently tested, there are now tests that make sure
the following attributes are sufficiently checked,

  * Tag_ABI_FP_rounding
  * Tag_ABI_FP_denormal
  * Tag_ABI_FP_exceptions
  * Tag_ABI_FP_user_exceptions
  * Tag_ABI_FP_number_model

Also in this commit, the -unsafe-fp-math flag has been augmented with the full
suite of flags Clang sends to LLVM when you pass -ffast-math to Clang. That is,
`-unsafe-fp-math' has been changed to `-enable-unsafe-fp-math -disable-fp-elim
-enable-no-infs-fp-math -enable-no-nans-fp-math -fp-contract=fast'

Change-Id: I35d766076bcbbf09021021c0a534bf8bf9a32dfc