[PATCH, committed] fix a number of pdp11 target test failures

[Paul Koning]

This fixes a number of test failures due to test cases 
that are too large for pdp11 and should be skipped.  Also
one test case that asks for alignment larger than what
pdp11 supports.

  paul


        2018-05-25 Paul Koning <n...@arrl.net> 

        * gcc.c-torture/compile/20151204.c: Skip if pdp11.
        * gcc.c-torture/compile/pr55921.c: Ditto.
        * gcc.c-torture/compile/pr60655-1.c: Ditto.
        * gcc.c-torture/compile/vector-align-1.c: Add max alignment if pdp11.