xtensa: Remove arch-specific strncpy() implementation

strncpy() has no remaining callers in the kernel[1]. Remove the
xtensa-specific inline assembly implementation and __HAVE_ARCH_STRNCPY
define, falling back to the generic version in lib/string.c.

Link: https://github.com/KSPP/linux/issues/90 [1]
Signed-off-by: Kees Cook <kees@kernel.org>