target-arm: Fix return address for A64 BRK instructions

When we take an exception resulting from a BRK instruction,
the architecture requires that the "preferred return address"
reported to the exception handler is the address of the BRK
itself, not the following instruction (like undefined
insns, and in contrast with SVC, HVC and SMC). Follow this,
rather than incorrectly reporting the address of the following
insn.

(We do get this correct for the A32/T32 BKPT insns.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Cc: qemu-stable@nongnu.org