Revert "[mips][compiler-rt] Provide 64bit atomic add and sub"

This reverts commit r321260. It appears to have broken the sanitizer
bot sanitizer-ppc64be-linux.

http://lab.llvm.org:8011/builders/sanitizer-ppc64be-linux/builds/5029

Reverting to see if the buildbot turns green.

llvm-svn: 321292