genmultilib: Fix sanity check

My previous patch to add a sanity check to genmultilib actually
checked the number of dirnames with the number of "sets of options"
rather than the number of options, thus breaking the build on some
targets.

To avoid duplicating once more the loop that constructs the sed
patterns, this patch checks that the current dirname/osdirname is not
empty in the existing loops.

Are there targets where:
if [ "$1" != "${opt}" ]; then
is "legally" executed with an empty $1? (and thus where this patch
would incorrectly trigger an error?)

Sorry for the breakage. Tested on aarch64 by adding an option to
t-aarch64 and no corresponding dirname, and on x86_64.

gcc/ChangeLog:

	* genmultilib: Fix options and dirnames/osdirnames sanity check.