Update build scripts for new target directory

In commit d7f12621565fbe60a7d369215a73f4a5b3610312 (svn r7392), the
files for each build target were relocated to build/lib/targets, so that
all target-specific files are kept in one place.

This change updates the scripts which manage adding new targets and
building cross-compiled regression tests, since they are sensitive to
the target directory structure.
Update regression suite for new assembly suffix

As of commit 1dfc86574fb41ae24ceb23d624f5606be661a9b6 (SVN r7098), MLton
produces assembly files with the '.s' extension, rather than '.S', since
no preprocessing needs to be done one them.

This change updates the regression test script to look for '.s' files in
addition to '.S' files when performing the second half of building a
cross-compiled regression test suite on the target machine.
Merge pull request #13 from caltry/fix-cross-build

Cross compilation fixes

While working through http://www.mlton.org/PortingMLton, I ran into a couple of problems with adding targets and running the regression test suite. The problems were related to changes in the directory structure of `build/` and the suffixes used for generated assembly files.

This PR fixes:

- Adding a new target to MLton (via `bin/add-cross`).
- Running a cross-compiled regression suite (via `bin/regression -run-only <target>`).