x86: add %gs accessors for x86_32

Impact: cleanup

On x86_32, %gs is handled lazily.  It's not saved and restored on
kernel entry/exit but only when necessary which usually is during task
switch but there are few other places.  Currently, it's done by
calling savesegment() and loadsegment() explicitly.  Define
get_user_gs(), set_user_gs() and task_user_gs() and use them instead.

While at it, clean up register access macros in signal.c.

This cleans up code a bit and will help future changes.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/math-emu/get_address.c

@@ -150,11 +150,9 @@ static long pm_address(u_char FPU_modrm, u_char segment,
 #endif /* PARANOID */
 
 	switch (segment) {
-		/* gs isn't used by the kernel, so it still has its
-		   user-space value. */
 	case PREFIX_GS_ - 1:
-		/* N.B. - movl %seg, mem is a 2 byte write regardless of prefix */
-		savesegment(gs, addr->selector);
+		/* user gs handling can be lazy, use special accessors */
+		addr->selector = get_user_gs(FPU_info->regs);
 		break;
 	default:
 		addr->selector = PM_REG_(segment);