exec.c: Capture the memory attributes for a watchpoint hit

Capture the memory attributes for the transaction which triggered
a watchpoint; this allows CPU specific code to implement features
like ARM's "user-mode only WPs also hit for LDRT/STRT accesses
made from privileged code". This change also correctly passes
through the memory attributes to the underlying device when
a watchpoint access doesn't hit.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
