arch/arm: Fix gcc-9.2 warning with z_arch_syscall_invoke6

Fix the following warning from gcc-9.2:

arm/syscall.h:52:2: error: listing the stack pointer register 'sp'
 in a clobber list is deprecated [-Werror=deprecated]
   52 |  __asm__ volatile("svc %[svid]\n"
      |  ^~~~~~~

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>