[ELF] - Ignore --plugin-opt=mcpu= and --plugin-opt=thinlto.

Attemp to fix BB.

Previously we ignored all --plugin-opt options, 
but now (after r310826) error out on non implemented ones.
This caused BB to fail:

http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux/builds/3405/steps/check-cfi-and-supported%20in%20gcc%20build/logs/stdio

llvm-svn: 310835