Because pre-v6 ARM lacks support for an atomic compare-and-swap, we
implement _lock_cas() as a restartable atomic squence that is checked
in the IRQ handler right before AST processing. (This is safe because,
for all practical purposes, there are no SMP pre-v6 ARM systems.)
This can serve as a model for other non-MP platforms that lack the
necessary atomic operations for mutexes (SuperH, for example).
Upshots of this change:
- kmutex_t is now down to 8 bytes on ARM; about as good as we can get.
- ARM2 systems don't have to trap and emulate SWP or SWPB for mutexes.
The acorn26 port is not updated by this commit to do the LOCK_CAS_CHECK.
That is left as an exercise for the port maintainer.
Reviewed and tested by Matt Thomas.
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