Rebase of #610 and also move emscripten up much higher in the hierarchy to
ensure that it doesn't have too much of a ripple effect on other platforms.

This involved moving down a good number of definitions, but hopefully was done
with care to not break anything!

We can't test s390x because qemu segfaults but we can at least verify that it
compiles.

Closes #650

Forgot to update cargo.lock before prior commit.

closes https://github.com/rust-lang/libc/issues/539

Also cache on appveyor to speed up builds.

in order to advise contributors to locally test their patches.

Also update ctest to include a fix on rerun-if-changed so that human
developers doing trial & error can properly test their latest code.

Signed-off-by: NODA, Kai <nodakai@gmail.com>