[ARM] support symbolic expression as immediate in memory instructions

Currently the ARM backend only accpets constant expressions as the
immediate operand in load and store instructions. This allows the
result of symbolic expressions to be used in memory instructions. For
example,

0:
.space 2048
strb r2, [r0, #(.-0b)]

would be assembled into the following instructions.

strb	r2, [r0, #2048]

This only adds support to ldr, ldrb, str, and strb in arm mode to
address the build failure of Linux kernel for now, but should facilitate
adding support to similar instructions in the future if the need arises.

Link:
https://github.com/ClangBuiltLinux/linux/issues/1329

Reviewed By: peter.smith, nickdesaulniers

Differential Revision: https://reviews.llvm.org/D98916