[AArch64] Fix a bug with jump table generation

Summary:
When trying to calculate the offsets for the jump table entries
we fail to take into account the block alignment, which could be
greater than 4 bytes. This led to cases where the jump table
offset was too big to fit in a byte.

Reviewers: t.p.northover, sdesmalen, ostannard

Reviewed By: ostannard

Subscribers: ostannard, kristof.beyls, hiraditya, llvm-commits

Committed on behalf of David Sherwood (david-arm)

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70533