x86: use kernel_stack_pointer() in kgdb.c

The way to obtain a kernel-mode stack pointer from a struct
pt_regs in 32-bit mode is "subtle": the stack doesn't actually
contain the stack pointer, but rather the location where it would
have been marks the actual previous stack frame.  For clarity, use
kernel_stack_pointer() instead of coding this weirdness
explicitly.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Cc: Jason Wessel <jason.wessel@windriver.com>