[lld-macho] Handle LOH_ARM64_ADRP_LDR linker optimization hints

This linker optimization hint transforms a pair of adrp+ldr (immediate)
instructions into an ldr (literal) load from a PC-relative address if
it is 4-byte aligned and within +/- 1 MiB, as ldr can encode a signed
19-bit offset that gets multiplied by 4.

In the wild, only a small number of these hints are applicable because
not many loads end up close enough to the data segment. However, the
added helper functions will be useful in implementing the rest of the
LOH types.

Differential Revision: https://reviews.llvm.org/D128942