[ARM] Add target feature to force 32-bit atomics

This adds a +atomic-32 target feature, which instructs LLVM to assume
that lock-free 32-bit atomics are available for this target, even
if they usually wouldn't be.

If only atomic loads/stores are used, then this won't emit libcalls.
If atomic CAS is used, then the user is responsible for providing
any necessary __sync implementations (e.g. by masking interrupts
for single-core privileged use cases).

See https://reviews.llvm.org/D120026#3674333 for context on this
change. The tl;dr is that the thumbv6m target in Rust has
historically made atomic load/store only available, which is
incompatible with the change from D120026, which switched these to
use libatomic.

Differential Revision: https://reviews.llvm.org/D130480