Start cleaning up libm.

We have two copies of fenv.h for every architecture, one of which
isn't used. We also have unused makefiles and files for architectures
we don't support.

This patch removes all the obviously useless files.

Bug: http://code.google.com/p/android/issues/detail?id=38196
Change-Id: I1919b6621ba513aa24aa947a34815bc51191487c