[AArch64] Support expression results as immediate values in mov

Summary:
This patch adds support of using the result of an expression as an
immediate value. For example,

0:
.skip 4
 1:
mov x0, 1b - 0b

is assembled to

mov x0, #4

Currently it does not support expressions requiring relocation unless
explicitly specified. This fixes PR#45781.

Reviewers: peter.smith, ostannard, efriedma

Reviewed By: efriedma

Subscribers: nickdesaulniers, llozano, manojgupta, efriedma, ostannard, kristof.beyls, hiraditya, danielkiss, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80028