Add "-mcpu=" option to llvm-rtdyld

This patch adds the -mcpu= option to llvm-rtdyld. With this option, one
can test relocations for different types of CPUs (e.g. Mips64r6).

Patch by Vladimir Radosavljevic.

Differential Revision: http://reviews.llvm.org/D10503

llvm-svn: 240477