Configure and testsuite cleanups, v2

The configure changes I hope will make for better support when building inside gcc.
In particular, gcc's top-level configure can source configure.host to see if the libffi
directory is supported for the target. C.f. the similar structure for gcc/libatomic.

The testsuite changes fix both missing tests, and the near total failure using clang.
It also removes the need to adjust the testsuite when support for new features is
added for targets; the additional tests will check for the feature defines in the header.