[MC] Fallback on DWARF when generating compact unwind on AArch64

Instead of asserting when using the def_cfa directive with a register
different from fp, fallback on DWARF.

Easily triggered with:

.cfi_def_cfa x1, 32;

rdar://40249694

Differential Revision: https://reviews.llvm.org/D47593

llvm-svn: 333667