[ARM] Lower unaligned loads/stores to aeabi functions. (#172672)

When targeting architectures that do not support unaligned memory
accesses or when explictly pass -mno-unaligned-access, it requires the
compiler to expand each unaligned load/store into an inline sequences.
For 32-bit operations this typically involves:

	1. 4× LDRB (or 2× LDRH),
	2. multiple shift/or instructions

These sequences are emitted at every unaligned access site, and
therefore contribute significant code size in workloads that touch
packed or misaligned structures.

When compiling with -Oz and in combination with -mno-unaligned-access,
this patch lowers unaligned 32 bit and 64 bit loads and stores to below
AEABI heper calls:
```
         __aeabi_uread4
	__aeabi_uread8
	__aeabi_uwrite4
	__aeabi_uwrite8
```

And it provide a way to perform unaligned memory accesses on targets
that do not support them, such as ARMv6-M or when compiling with
-mno-unaligned-access. Although each use introduces a function call
making it less straightforward than using raw loads and stores the call
itself is often much smaller than the compiler emitted sequence of
multiple ldrb/strb operations. As a result, these helpers can greatly
reduce code-size providing they are invoked more than once across a
program.

1. Functions become smaller in AEABI mode once they contain more than a
few unaligned accesses.
2. The total image .text size becomes smaller whenever multiple
functions call the same helpers.

This PR is derived from https://reviews.llvm.org/D57595, with some minor
changes.
 Co-authored-by: David Green