Remove a use of OPT_m_Joined.

This patch turns the -mv* hexagon options into aliases. We should really produce
errors for invalid versions in the driver, but this patch preserves the old
behavior for now.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191298 91177308-0d34-0410-b5e6-96231b3b80d8