target/arm: adjust program counter for wfi exception in AArch32

The wfi instruction can be configured to be trapped by a higher exception
level, such as the EL2 hypervisor. When the instruction is trapped, the
program counter should contain the address of the wfi instruction that
caused the exception. The program counter is adjusted for this in the wfi op
helper function.

However, this correction is done to env->pc, which only applies to AArch64
mode. For AArch32, the program counter is stored in env->regs[15]. This
adds an if-else statement to modify the correct program counter location
based on the the current CPU mode.

Signed-off-by: Jeff Kubascik <jeff.kubascik@dornerworks.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>