UPSTREAM: ARM: 8170/1: Add global named register current_stack_pointer for ARM

Define a global named register for current_stack_pointer. The use of this new
variable guarantees that both gcc and clang can access this register in C code.

Signed-off-by: Behan Webster <behanw@converseincode.com>
Reviewed-by: Jan-Simon Möller <dl9pf@gmx.de>
Reviewed-by: Mark Charlebois <charlebm@gmail.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Acked-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
(cherry picked from commit 0abc08baf2ddf61a37d375a9fb832be612f9c5a5)
Bug: 29520177
Signed-off-by: Mohan Srinivasan <srmohan@google.com>
Change-Id: I35d828dc5787d5ec698ec4fba5cbc352805f92e8