Rename unroll_[1-5].c

[Mike Stump]

So, I wanted to add some unrolling test cases and found that we had unroll-1.c 
and unroll_[1-5].c.  :-(  - is the standard, and picking numbers sequential 
from 1, with a - before the number is standard.  No reason to deviate in this 
case.  I’ve fixed this by renaming the test cases like so:

        * gcc.dg/unroll_1.c: Rename gcc.dg/unroll_[1-5].c to unroll-[2-6].
        * gcc.dg/unroll_2.c: Likewise.
        * gcc.dg/unroll_3.c: Likewise.
        * gcc.dg/unroll_4.c: Likewise.
        * gcc.dg/unroll_5.c: Likewise.