syscall: Use a more compatible writing style

compile error:
Register: ostest
Register: nsh
Register: sh
Register: hello
Register: getprime
In file included from /home/hujun5/downloads1/vela_sim/nuttx/include/arch/irq.h:35,
                 from /home/hujun5/downloads1/vela_sim/nuttx/include/nuttx/irq.h:37,
                 from /home/hujun5/downloads1/vela_sim/nuttx/include/nuttx/sched.h:40,
                 from /home/hujun5/downloads1/vela_sim/nuttx/include/nuttx/arch.h:87,
                 from common/arm_signal_dispatch.c:26:
common/arm_signal_dispatch.c: In function 'up_signal_dispatch':
common/arm_signal_dispatch.c:72:3: error: 'asm' operand has impossible constraints
   72 |   sys_call4(SYS_signal_handler, (uintptr_t)sighand, (uintptr_t)signo,
      |   ^~~~~~~~~
make[1]: *** [Makefile:168：arm_signal_dispatch.o] error 1

Change-Id: Ib183175366361f0228ed3607fcb2193b38cfd672