[ELF/AARCH64] - Implemented R_AARCH64_CONDBR19 relocation.

R_AARCH64_CONDBR19 is calculated as S+A-P,
Set the immediate field of a conditional branch instruction to bits [20:2] of X; check -2^20 ≤ X< 2^20.

Afaik there is no document for AARCH64 instruction encoding from official for unknown reason, so 
I used gold source code and next link as a reference for implementation: http://kitoslab-eng.blogspot.ru/2012/10/armv8-aarch64-instruction-encoding.html. From which is clear that immediate field of a conditional branch instruction is 5 bits off. That is proved by output which is is equal to gold/bfd now.

Differential revision: http://reviews.llvm.org/D15809

llvm-svn: 257333