ARM: 8174/1: Use global stack register variable for percpu

Using global current_stack_pointer works on both clang and gcc.
current_stack_pointer is an unsigned long and needs to be cast
as a pointer to dereference.

Signed-off-by: Mark Charlebois <charlebm@gmail.com>
Signed-off-by: Behan Webster <behanw@converseincode.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>