target-arm/translate-a64: fix gen_load_exclusive

target-arm/translate-a64: fix gen_load_exclusive

While testing rth's latest TCG patches with risu I found ldaxp was
broken. Investigating further I found it was broken by 1dd089d0 when
the cmpxchg atomic work was merged. As part of that change the code
attempted to be clever by doing a single 64 bit load and then shuffle
the data around to set the two 32 bit registers.

As I couldn't quite follow the endian magic I've simply partially
reverted the change to the original code gen_load_exclusive code. This
doesn't affect the cmpxchg functionality as that is all done on in
gen_store_exclusive part which is untouched.

I've also restored the comment that was removed (with a slight tweak
to mention cmpxchg).

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Acked-by: Richard Henderson <rth@twiddle.net>
Message-id: 20161202173454.19179-1-alex.bennee@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>