[lld][ELF] Support adrp+ldr GOT optimization for AArch64

This diff adds first bits to support relocation relaxations for AArch64
discussed on https://github.com/ARM-software/abi-aa/pull/106.
In particular, the case of

adrp x0, :got: symbol
ldr x0, [x0, :got_lo12: symbol]

is handled.

Test plan: make check-all

Differential revision: https://reviews.llvm.org/D112063