serial: earlycon-arm-semihost: Move smh_putc() variants in respective arch's semihost.h

Move smh_putc() variants in respective arch/*/include/asm/semihost.h,
in preparation to add RISC-V support.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Tested-by: Sergey Matyukevich <sergey.matyukevich@syntacore.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Link: https://lore.kernel.org/r/20221209150437.795918-2-bmeng@tinylab.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>