target-arm: ignore ELR_ELx[1] for exception return to 32-bit ARM mode

The architecture requires that for an exception return to AArch32 the
low bits of ELR_ELx are ignored when the PC is set from them:
 * if returning to Thumb mode, ignore ELR_ELx[0]
 * if returning to ARM mode, ignore ELR_ELx[1:0]

We were only squashing bit 0; also squash bit 1 if the SPSR T bit
indicates this is a return to ARM code.

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

target-arm/op_helper.c

@@ -738,7 +738,11 @@ void HELPER(exception_return)(CPUARMState *env)
         }
         aarch64_sync_64_to_32(env);
 
-        env->regs[15] = env->elr_el[cur_el] & ~0x1;
+        if (spsr & CPSR_T) {
+            env->regs[15] = env->elr_el[cur_el] & ~0x1;
+        } else {
+            env->regs[15] = env->elr_el[cur_el] & ~0x3;
+        }
     } else {
         env->aarch64 = 1;
         pstate_write(env, spsr);