Rollup merge of #99595 - nikic:thumbv6m-atomics, r=nagisa

Rollup merge of #99595 - nikic:thumbv6m-atomics, r=nagisa

Mark atomics as unsupported on thumbv6m

The thumbv6m target does not support atomics. Historically, LLVM
had a bug where atomic load/stores for this target were emitted
as plain load/stores rather than as libatomic calls. This was
fixed in https://reviews.llvm.org/D120026, which will be part of
LLVM 15. As we require that "atomic support" does not use libatomic,
we need to indicate that this target does not have native atomics.