Fixes Ravenbrook#168.

When compiling with `-s USE_PTHREADS` in emscripten, Posix threads
and locks are available (assuming the WebAssembly VM supports this
extension).

This can be detected at compile time by checking for the preprocessor
definition `__EMSCRIPTEN_PTHREADS__`.

In the absence of this definition, we request a single threaded
configuration which falls back to the implementation in `lockan.c`.

This works when building `mps.c`, but does not work when
using the older Makefile-based build system as when building
`lockix.c` as a standalone file, `mpstd.h` is not included prior
to the relevant portion of `config.h` being processed.

This uses GitHub actions and installs the Emscripten SDK's
latest version and activates it. At some point, this should
probably change to a fixed version that is periodically
incremented once some baseline support level is determined.

This is just compiling and running what's needed for `amsss`
for now until other pieces are in place for building and
running more of everything.

This fails still due to threaded tests being built and that
failing.

Note that this is only valid as noted in a fixme comment when
threads are enabled in emscripten (which is not the default).

WebAssembly doesn't support `mprotect()` or signals. This will
make it hard to support multithreaded or incremental collection
for now.