Reapply r304929 [mips] Add runtime options to enable/disable madd/sub.fmt

Reapply r304929 [mips] Add runtime options to enable/disable madd/sub.fmt

The test in r304929 broke multiple buildbots as it expected mips target to
be registered and available (which is not necessarily true). Updating the
test with this condition.

Original commit:

  [mips] Add runtime options to enable/disable madd.fmt and msub.fmt

  Add options to clang: -mmadd4 and -mno-madd4, use it to enable or disable
  generation of madd.fmt and similar instructions respectively, as per GCC.

  Patch by Stefan Maksimovic.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@304953 91177308-0d34-0410-b5e6-96231b3b80d8