fix the following 'make headers_check' warning:

  usr/include/asm/prctl.h:10: extern's make no sense in userspace
Signed-off-by: NJaswinder Singh Rajput <jaswinderrajput@gmail.com>

This reverts commit 4217458d.

Justin Madru bisected this commit, it was causing weird Firefox
crashes.

The reason is that GCC mis-optimizes (re-uses) the on-stack parameters of
the calling frame, which corrupts the syscall return pt_regs state and
thus corrupts user-space register state.

So we go back to the slightly less clean but more optimization-safe
method of getting to pt_regs. Also add a comment to explain this.

Resolves: http://bugzilla.kernel.org/show_bug.cgi?id=12505Reported-and-bisected-by: NJustin Madru <jdm64@gawab.com>
Tested-by: NJustin Madru <jdm64@gawab.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Impact: cleanup

In asm/syscalls.h move out sys_set_thread_area() and sys_get_thread_area()
as they are common for both 32 and 64 bit.
Signed-off-by: NJaswinder Singh <jaswinder@infradead.org>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Impact: cleanup

In asm/syscalls.h moved out sys_modify_ldt from CONFIG_X86_32 as it is
common for both 32 and 64 bit.
Signed-off-by: NJaswinder Singh <jaswinder@infradead.org>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Impact: cleanup on 32-bit

Peter pointed this parameter can be changed.
Signed-off-by: NHiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>
Signed-off-by: NH. Peter Anvin <hpa@zytor.com>

Signed-off-by: NJaswinder Singh <jaswinder@infradead.org>

declared following syscalls in asm-x86/syscalls.h:
sys_vm86old, sys_vm86
Signed-off-by: NJaswinder Singh <jaswinder@infradead.org>

Declaring arch-dependent syscalls for x86 architecture
Signed-off-by: NJaswinder Singh <jaswinder@infradead.org>