x86/entry/32: Enable pt_regs based syscalls

Enable pt_regs based syscalls for 32-bit.  This makes the 32-bit native
kernel consistent with the 64-bit kernel, and improves the syscall
interface by not needing to push all 6 potential arguments onto the stack.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Dominik Brodowski <linux@dominikbrodowski.net>
Link: https://lkml.kernel.org/r/20200313195144.164260-17-brgerst@gmail.com