constant: don't skip the E2BIG test on Hurd, but improve comments

constant: don't skip the E2BIG test on Hurd, but improve comments

As Father Chrysostomos explained on [perl #128319] the previous change
was based on a misunderstanding of the purpose of the test, which is
only to test that constant.pm can create a dualvar out of $! which
has an error string, and nothing specifically to do with E2BIG.

The new comments, also suggested by Father Chrysostomos, make this more
clear.