89582e8193
https://reviews.llvm.org/D120026 changed atomics on thumbv6m to use libatomic, to ensure that atomic load/store are compatible with atomic RMW/CAS. However, Rust wants to expose only load/store without libcalls. https://reviews.llvm.org/D130480 added support for this behind the +atomics-32 target feature, so enable that feature.
rustc_target contains some very low-level details that are
specific to different compilation targets and so forth.
For more information about how rustc works, see the rustc dev guide.