x86/entry/clearregs: Clear registers for 32bit kernel

On a 32bit kernel clearing registers is much simpler than
on 64bit. The arguments for syscalls are initially passed
to a C function through the stack, so there's no need
to figure out how many arguments to clear.

So we always clear all registers (except frame pointer) for
all entry points.

Signed-off-by: Andi Kleen <ak@linux.intel.com>