atomic(9): Implement atomic_testand(clear|set)_ptr

[jhb]

Summary

For current architectures, these are just aliases for the existing
operation on the relevant scalar integer.

Obtained from: CheriBSD
Sponsored by: AFRL, DARPA

[imp]

All look good except I'm not so sure about 32bit powerpc

[jhb]

Should have noted that this passes make universe (and I've also booted it on some architectures).

sys/powerpc/include/atomic.h

1100

This matches the pattern of other 32-bit powerpc ops of using a cast instead of just mapping the macro directly.