Remove memory barriers from the atomic_ops(3) atomic operations. They're
not needed for correctness.

Add the correct memory barriers to the gcc legacy __sync built-in
functions for atomic memory access. From the gcc documentation:

In most cases, these built-in functions are considered a full barrier.
That is, no memory operand is moved across the operation, either forward
or backward. Further, instructions are issued as necessary to prevent the
processor from speculating loads across the operation and from queuing
stores after the operation.

type __sync_lock_test_and_set (type *ptr, type value, ...)

This built-in function is not a full barrier, but rather an acquire
barrier. This means that references after the operation cannot move to
(or be speculated to) before the operation, but previous memory stores
may not be globally visible yet, and previous memory loads may not yet
be satisfied.

void __sync_lock_release (type *ptr, ...)

This built-in function is not a full barrier, but rather a release
barrier. This means that all previous memory stores are globally
visible, and all previous memory loads have been satisfied, but
following memory reads are not prevented from being speculated to
before the barrier.

s/ifdef _ARM_ARCH_6/if defined(_ARM_ARCH_6)/ for consistency. NFCI.

Add the appropriate memory barrier before the lock is cleared in
__sync_lock_release_{1,2,4,8}. That is, all reads and write for in inner
shareability domain before the lock clear store.

Trailing whitespace

Fix __sync_lock_release_4 to actually zeroise the whole 4bytes/32bits.

branches: 1.14.8; 1.14.16; 1.14.18;
Do not build arm toolchain symbols in the rump kernel

These symbols will be provided at link time and will be
duplicate symbols in rump kernel and libc if defined.

Many have been fixed previously, but these were missed
as did not have a test.

Use the right register in previous. Spotted by matt@

ARM ARM D7.3.2 - ensure all previous accesses are observed before
the lock is cleared

Add aliases for the C11/C++11 spelling of the CAS primitives.

branches: 1.10.2;
Don't export __sync* if _KERNEL || _STANDALONE are defined.
(except if _RUMPKERNEL is defined)

switch to unified syntax

Add support for the gcc __sync builtins.
Note that these need earmv6 or later to get the ldrex/strex instructions

Use foo{s} since it reduces the ifdefs for thumb

Make these under Thumb2

Change movsne to movnes for clang.

branches: 1.4.2;
Add dmb/dsb instructions as required by the armv7 arch man.

Actually use the assembly version of the atomic function if compiling
for armv6 or armv7 cpus. Use atomic_cas_ptr instead of _lock_cas so
we pick up the assembly version when it's used.

branches: 1.2.2; 1.2.4; 1.2.12; 1.2.24;
Add assembly versions of atomic ops with ldrex/strex

branches: 1.1.2;
file atomic_swap.S was initially added on branch thorpej-atomic.