strncpy() has no remaining callers in the kernel[1]. Remove the
alpha-specific assembly implementation and __HAVE_ARCH_STRNCPY define,
falling back to the generic version in lib/string.c.
The __stxncpy helper (stxncpy.S/ev6-stxncpy.S) is retained as it is
still used by strncat.
Link: https://github.com/KSPP/linux/issues/90 [1]
Signed-off-by: Kees Cook <kees@kernel.org>