[JITLink][ELF][AArch64] Implement Procedure Linkage Table.

[JITLink][ELF][AArch64] Implement Procedure Linkage Table.

Implements Procedure Linkage Table (PLT) for ELF/AARCH64. The aarch64 linux calling convention also uses r16 as the intra-procedure-call scratch register same as MachO/ARM64. We can use the same stub sequence for this reason.

Also, BR regiseter doesn't touch X30 register. External function call by BL instruction (touched by CALL26 relocation) will set X30 to the original PC + 4, which is the intended behavior. External function call by B instruction (touched by JUMP26 relocation) doesn't requite to set X30, so the patch will be correct in this case too.

Reference: https://github.com/ARM-software/abi-aa/blob/main/aapcs64/aapcs64.rst#611general-purpose-registers

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D127061