target-arm: A64: support for ld/st/cl exclusive

This implement exclusive loads/stores for aarch64 along the lines of
arm32 and ppc implementations. The exclusive load remembers the address
and loaded value. The exclusive store throws an an exception which uses
those values to check for equality in a proper exclusive region.

This is not actually the architecture mandated semantics (for either
AArch32 or AArch64) but it is close enough for typical guest code
sequences to work correctly, and saves us from having to monitor all
guest stores. It's fairly easy to come up with test cases where we
don't behave like hardware - we don't for example model cache line
behaviour. However in the common patterns this works, and the existing
32 bit ARM exclusive access implementation has the same limitations.

AArch64 also implements new acquire/release loads/stores (which may be
either exclusive or non-exclusive). These imposes extra ordering
constraints on memory operations (ie they act as if they have an implicit
barrier built into them). As TCG is single-threaded all our barriers
are no-ops, so these just behave like normal loads and stores.

Signed-off-by: Michael Matz <matz@suse.de>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
