lib: utils/serial: Optimize semihosting_putc implementation

For some debuggers that do not implement SYSWRITEC and SYSREADC
operations, we can use SYSWRITE and SYSREAD instead like the
implementation of semihosting_getc().

Signed-off-by: Chen Pei <cp0613@linux.alibaba.com>
Reviewed-by: Xiang W <wxjstz@126.com>
Reviewed-by: Anup Patel <anup@brainfault.org>