[Clang] Default to async unwind tables for amdgcn
To avoid codegen changes when enabling debug-info (see
https://bugs.llvm.org/show_bug.cgi?id=37240) we want to
enable unwind tables by default.
There is some pessimization in post-prologepilog scheduling, and a
general solution to the problem of CFI_INSTRUCTION-as-scheduling-barrier
should be explored.
Change-Id: I83625875966928c7c4411cd7b95174dc58bda25a
