Fix GCC 8's -Wstringop-overflow on bfd/coff-rs6000.c

Fix GCC 8's -Wstringop-overflow on bfd/coff-rs6000.c

GCC 8 will bring a new warning option which will detect possible
overflow and truncation on string manipulation functions.  For more
details, see:

  https://gcc.gnu.org/ml/gcc-patches/2017-08/msg00471.html

While compiling BFD with it, I can see one place on bfd/coff-rs6000.c
where the warning is triggered.  This:

  (void) strncpy (fhdr.magic, XCOFFARMAG, SXCOFFARMAG);

will not include the trailing NUL on fhdr.magic, but that's fine
because it's a magic number.  The fix is trivial: just use memcpy
instead.

OK to push?

2018-02-06  Sergio Durigan Junior  <sergiodj@redhat.com>

	* coff-rs6000.c (xcoff_write_archive_contents_old): Use
	'memcpy' instead of 'strncpy' when writing the magic number.