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.
CodeGen: add sensible defaults for the ISD::FROUND operation

Some exotic types didn't know how to handle FROUND, which ARM64 uses.
TableGen: avoid dereferencing nullptr variable

ARM64 ended up reaching odder parts of TableGen alias generation than
current backends and caused a segfault.