Fix -Os strcoll, wcscoll, build (bug 21313).

The findidx functions used in implementing strcoll / wcscoll already
use DIAG_IGNORE_Os_NEEDS_COMMENT for spurious -Wmaybe-uninitialized
warnings that appear with -Os.  In building with GCC 7 for x86_64 with
-Os, I find there are additional such warnings, for the same structure
elements, which are spurious for the same reasons given in the
existing comments (and this was also reported for MIPS with GCC 5 in
bug 21313).  This patch adds corresponding uses of DIAG_* in the
places that get the additional warnings.

Tested for x86_64 with -Os that this eliminates those warnings and so
allows the build to progress further.

	[BZ #21313]
	* locale/weight.h (findidx): Disable -Wmaybe-uninitialized for -Os
	in another place.
	* locale/weightwc.h (findidx): Likewise.