powerpc: Remove arch-specific strncpy() implementation

powerpc: Remove arch-specific strncpy() implementation

strncpy() has no remaining callers in the kernel[1]. Remove the
powerpc-specific assembly implementation from both the kernel
(arch/powerpc/lib/string.S) and the boot wrapper
(arch/powerpc/boot/string.S), along with the __HAVE_ARCH_STRNCPY
define and declaration, falling back to the generic version in
lib/string.c.

The boot wrapper's strncpy had no callers in arch/powerpc/boot/.

Link: https://github.com/KSPP/linux/issues/90
Signed-off-by: Kees Cook <kees@kernel.org>