rework sgi mutexes to use the owner pointer as the lock.

rework sgi mutexes to use the owner pointer as the lock.

this is like src/sys/arch/alpha/alpha/mutex.c r1.14.

this changes sgi mutexes so they record which cpu owns the lock
rather than just if the lock is held or not. the diagnostics compare
the owner to the current cpus curcpu() address so they can actually
tell if the current cpu holds the lock instead of whether any cpu
holds the lock.

instead of using custom asm to implement a cas this uses atomic_cas_ptr.

while im here i also shuffled the code. on MULTIPROCESSOR systems
instead of duplicating code between mtx_enter and mtx_enter_try,
mtx_enter simply loops on mtx_enter_try until it succeeds.

this also provides an alternative implementation of mutexes on
!MULTIPROCESSOR systems that avoids interlocking opcodes. mutexes
wont contend on UP boxes, theyre basically wrappers around spls.
we can just do the splraise, stash the owner as a guard value for
DIAGNOSTIC and return. similarly, mtx_enter_try on UP will never
fail, so we can just call mtx_enter and return 1.

tested by and ok miod@