In almost all cases it is preferable to use the stable `asm!` instead of
calling these intrinsics.
This PR removes the following unstable intrinsics:
- `__breakpoint` Clang extension, not part of ACLE.
- `brk`: undocumented
- `_rev*`, `_clz*`, `_rbit*`: use methods on integer types instead
- `__ldrex`, `__strex`: deprecated in ACLE, hard to use correctly
- Register access API: API doesn't match ACLE, better to just use asm
Also considered for deletion, but not included in this PR:
- Barriers: `__isb`, `__dsb`, `__dmb`
- Hints: `__wfi`, `__wfe`, `__sev`, `__sevl`, `__yield`, `__nop`
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