ARMv7k: select ABI based on v7k Arch rather than watchos OS.

Various bits we'd like to use the new ABI actually compile with "-arch armv7k
-miphoneos-version-min=9.0". Not ideal, but also not ridiculous given how
slices work.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@258976 91177308-0d34-0410-b5e6-96231b3b80d8