[AMDGPU] Add default ISA version targets

In case if -mattr used to modify feature set bits in llvm-mc call
getIsaVersion can fail to identify specific ISA due to test mismatch.
Adding default fallback tests which will always correctly report at
least major version.

Differential Revision: https://reviews.llvm.org/D44163

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@326825 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed