Use portable-atomic crate on no-std target

[taiki-e]

This allows removing volatile read/write hack on targets do not support atomic load/store.

atomic-memcpy/src/lib.rs

Lines 832 to 834 in 6a2a8f6

	// HACK: Using volatile read/write instead of atomic load/store on single-threaded platforms where
	// LLVM does not support atomic is normally considered to be an okay workaround.
	// <https://github.com/rust-lang/compiler-builtins/commit/e0187f17dbcbf9dc026d379b2af8d866300596a5>

refs: portable-atomic

[taiki-e]

This will reduce performance somewhat for types with smaller alignments, but considering that the current one is technically UB, that atomic load/store support will be added in LLVM on these platforms, and that after the standard library supports it, portable-atomic's version detection can handle them, tend to merge this.

Users who need the same codegen as before can set up an unsafe cfg atomic_memcpy_unsafe_volatile as before.

[taiki-e]

bors r+

[bors]

Build succeeded:

• ci