powerpc: Add current_stack_pointer as a register global

current_stack_frame() doesn't return the stack pointer, but the
caller's stack frame. See commit bfe9a2cfe91a ("powerpc: Reimplement
__get_SP() as a function not a define") and commit
acf620ecf56c ("powerpc: Rename __get_SP() to current_stack_pointer()")
for details.

In some cases this is overkill or incorrect, as it doesn't return the
current value of r1.

So add a current_stack_pointer register global to get the value of r1
directly.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
[mpe: Split out of other patch, tweak change log]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200220115141.2707-2-mpe@ellerman.id.au