genmultilib: Add sanity check

When a list of dirnames is provided to genmultilib, its length is
expected to match the number of options.  If this is not the case, the
build fails later for reasons not obviously related to this mistake.
This patch adds a sanity check to help diagnose such cases.

Tested by adding an option to t-aarch64 and no corresponding dirname,
with both bash and dash.

v2: do not use arrays (bash feature).

OK for trunk?

gcc/ChangeLog:

	* genmultilib: Add sanity check.