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.

Signed-off-by: Wilfried Chauveau <wilfried.chauveau@arm.com>
Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>