BaseTools: Add AArch64 ADR_PREL_LO21 and R_AARCH64_CONDBR19

BaseTools: Add AArch64 ADR_PREL_LO21 and R_AARCH64_CONDBR19
 relocations

- ADR_PREL_LO21: support for loading a PC relative label offset.
- R_AARCH64_CONDBR19: support for conditional branch instruction (ELF64 code: 280).

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Harry Liebel <Harry.Liebel@arm.com>
Signed-off-by: Olivier Martin <olivier.martin@arm.com>
Reviewed-by: Yingke Liu <yingke.d.liu@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15745 6f19259b-4bc3-4df7-8a09-765794883524