[SEH] Add initial support for AArch64

This doesn't yet implement inspecting the .pdata/.xdata to find the
LSDA pointer (in UnwindCursor::getInfoFromSEH), but normal C++
exception handling seems to run just fine without it. (The only
place I can see where it's even referenced is in
unwind_phase2_forced, and I can't find a codepath where libcxxabi
would end up calling that.)

Differential Revision: https://reviews.llvm.org/D55674

llvm-svn: 349532