powerpc/ps3: refactor strncpy usage

`strncpy` is deprecated for use on NUL-terminated destination strings [1].

`make_first_field()` should use similar implementation to `make_field()`
due to memcpy having more obvious behavior here. The end result yields
the same behavior as the previous `strncpy`-based implementation
including the NUL-padding.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: https://github.com/KSPP/linux/issues/90


Signed-off-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Tested-by: Geoff Levand <geoff@infradead.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230816-strncpy-arch-powerpc-platforms-ps3-repository-v1-1-88283b02fb09@google.com