Largely eliminate the MD rwlock.h header file.

This was full of definitions that have been obsolete for over a
decade. The file still remains for __HAVE_RW_STUBS but that's all.
Used only internally in kern_rwlock.c now, not by <sys/rwlock.h>.

branches: 1.9.18;
Improve inline asm around dsb/dmb/isb:
- always use volatile and mark them as memory barrier
- use the common version from locore.h in all places not included from
userland

branches: 1.8.2; 1.8.4;
Fix ARMv7 checks

_ARM_ARCH_7 && !_ARM_ARCH_6 never be true.

branches: 1.7.2;
Use dmb/dsb

Don't use _lock_cas explicitly, use atomic_cas_ulong which could be one its
aliases.

branches: 1.5.34; 1.5.44; 1.5.50;
Remove clause 3 and 4 from TNF licenses

branches: 1.4.14; 1.4.16; 1.4.18;
- Change _lock_cas() to do compare-and-swap instead of compare-and-set.
- Add aliases for atomic_cas_ulong() and friends.

make kmutex_t and krwlock_t smaller by killing lock id.
ok'ed by Andrew Doran.

branches: 1.2.2; 1.2.8; 1.2.16; 1.2.18; 1.2.20; 1.2.22; 1.2.26; 1.2.28;
Use _lock_cas() instead of the interrupt-blocking atomic_cas().

branches: 1.1.2; 1.1.6;
Add <arm/mutex.h> and <arm/rwlock.h>
Add atomic_cas to <arm/atomic.h> for use by <rwlock.h>