arch/x86: Fix gcc-9.2 warning with z_arch_syscall_invoke6

Fix the following warning from gcc-9.2:

x86/ia32/syscall.h: In function 'test_kinit_preempt_thread':
x86/ia32/syscall.h:43:2: error: listing the stack pointer register
 'esp' in a clobber list is deprecated [-Werror=deprecated]
   43 |  __asm__ volatile("push %%ebp\n\t"
      |  ^~~~~~~
x86/ia32/syscall.h:43:2: note: the value of the stack pointer after
an 'asm' statement must be the same as it was before the statement

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