Auto merge of #38579 - whitequark:min_atomic_width, r=alexcrichton

Add a min_atomic_width target option, like max_atomic_width

Rationale: some ISAs, e.g. OR1K, do not have atomic instructions
for byte and halfword access, and at the same time do not have
a fixed endianness, which makes it unreasonable to implement these
through word-sized atomic accesses.