exec.c: Add new address_space_ld*/st* functions

exec.c: Add new address_space_ld*/st* functions

Add new address_space_ld*/st* functions which allow transaction
attributes and error reporting for basic load and stores. These
are named to be in line with the address_space_read/write/rw
buffer operations.

The existing ld/st*_phys functions are now wrappers around
the new functions.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>