[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