s390: Fix Makefile.am. Essentially, revert r13317 which does not seem to
be necessary. The additional DIST_INSN_TESTS variable was causing problems
in check_makefile_consistecy which reported false errors. I suspect this
is because my awk is mawk and not gawk.
Also add dfp_utils.h which was missing in the tarball from 'make dist'.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13520 a5019735-40e9-0310-863c-91ae7b9d1cf9