[LLD][ELF][ARM] Fix case of patched unrelocated BLX

There are a couple of problems with the code to patch
unrelocated BLX instructions:
1. The calculation of the PC needs to take into account
   the alignment of the instruction. The Thumb BLX
   uses alignDown(PC, 4) for the source address.
2. The calculation of the PC bias is hard-coded to 4
   which works for Thumb, but when there is a BLX the
   branch will be in Arm state so it needs an 8 byte
   PC bias.

No asssembler generates an unrelocated BLX instruction
so these problems do not affect real world programs.
However we should still fix them.

Differential Revision: https://reviews.llvm.org/D104905