MC-exceptions: add support for compact-unwind without .eh_frame

ARM64 has compact-unwind information, but doesn't necessarily want to
emit .eh_frame directives as well. This teaches MC about such a
situation so that it will skip .eh_frame info when compact unwind has
been successfully produced.

For functions incompatible with compact unwind, the normal information
is still written.

llvm-svn: 205087