allow implementing a_cas_p with pointer-sized ll/sc

No current ports do this, but it will be useful for porting to 64-bit ll/sc
architectures, such as mips64 and powerpc64.

refactor internal atomic.h

rather than having each arch provide its own atomic.h, there is a new
shared atomic.h in src/internal which pulls arch-specific definitions
from arc/$(ARCH)/atomic_arch.h. the latter can be extremely minimal,
defining only a_cas or new ll/sc type primitives which the shared
atomic.h will use to construct everything else.

this commit avoids making heavy changes to the individual archs'
atomic implementations. definitions which are identical or
near-identical to what the new shared atomic.h would produce have been
removed, but otherwise the changes made are just hooking up the
arch-specific files to the new infrastructure. major changes to take
advantage of the new system will come in subsequent commits.

ensure that the compiler doesn't try to reorder around atomic ops

initial check-in, version 0.5.0