[AArch64] Add SEH_Nop opcodes for BTI hints

These are harmless for the unwinder - the unwinder doesn't need to
handle them for being able to unwind correctly.

Only add the opcodes when the branch target is in a SEH prologue;
for jumptables e.g. within a function, we shouldn't add any SEH
opcodes.

Differential Revision: https://reviews.llvm.org/D135277