default to use -mv4 when no version of Hexagon has been specified

This fixes a bunch of make check failures of the form:

Unknown Architecture Version.
UNREACHABLE executed at ../lib/Target/Hexagon/HexagonSubtarget.cpp:60!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@160518 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed