Revert "FROMPULL: arch: arm: cortex_m: move _main in input list"

This reverts commit 1307455.

Reason for revert: Commit landed upstream in zephyrproject-rtos/zephyr@7015a0e

Original change's description:
> FROMPULL: arch: arm: cortex_m: move _main in input list
>
> Move the _main argument to the input list rather than the output one on
> the asm block and change the spec to "r". The ASM block does not return,
> so it does not make sense for it to expect any output.
>
> zephyrproject-rtos/zephyr#80579
>
> BUG=b:376282914
>
> Change-Id: Ie6f1ab781bdc8d0de87353bcf48e2c4ade2d4dc1
> Signed-off-by: Wilfried Chauveau <wilfried.chauveau@arm.com>
> Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>
> Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/zephyr/+/5975475
> Tested-by: Jeremy Bettis <jbettis@chromium.org>
> Commit-Queue: Jeremy Bettis <jbettis@chromium.org>

BUG=b:376282914

Change-Id: I189e76cbfed650a98be338f9cd5c1aa79d396b47
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/zephyr/+/5991739
Auto-Submit: Yuval Peress <peress@google.com>
Commit-Queue: Yuval Peress <peress@google.com>
Reviewed-by: Fabio Baltieri <fabiobaltieri@google.com>
Tested-by: Yuval Peress <peress@google.com>

arch/arm/core/cortex_m/thread.c

@@ -586,8 +586,8 @@ void arch_switch_to_main_thread(struct k_thread *main_thread, char *stack_ptr,
 	"mov   r3, #0\n"
 	"ldr   r4, =z_thread_entry\n"
 	"bx    r4\n"		/* We don’t intend to return, so there is no need to link. */
-	:
-	: "r" (_main), "r" (stack_ptr)
+	: "+r" (_main)
+	: "r" (stack_ptr)
 	: "r0", "r1", "r2", "r3", "r4", "ip", "lr");
 
 	CODE_UNREACHABLE;