[ELF] - Allow set CPU string with -plugin-opt=mcpu=<xxx>

Previously we ignored -plugin-opt=mcpu=<xxx>
and the only way to set CPU string was to pass
-mllvm -mcpu=<xxx>
Though clang may pass it with use of plugin options:
-plugin-opt=mcpu=x86-64
Since we are trying to be compatible in command line
with gold plugin, seems we should support it too.

Differential revision: https://reviews.llvm.org/D42956

llvm-svn: 324459