Fix confused use of llvm::StringSwitch for armv7r architecture.

svn 170909 added support for cortex-r5 but in this case it was done
incorrectly. The last argument to StringSwitch.Cases() is the replacement
value, so by adding "cortex-r5" it changed the default cpu for armv7r to
cortex-r5 instead of cortex-r4.

llvm-svn: 176456