The xmlns attribute is required for older msbuild version (e.g. for
VS2015). Add it where needed, and reorder the attributes so all
files look the same.
No functionality change is intended with this commit, it just consolidates
the separate implementations of GC helper code to the lib/utils/ directory
as a general set of helper functions useful for any port. This reduces
duplication of code, and makes it easier for future ports or embedders to
get the GC implementation correct.
Ports should now link against gchelper_native.c and either gchelper_m0.s or
gchelper_m3.s (currently only Cortex-M is supported but other architectures
can follow), or use the fallback gchelper_generic.c which will work on
x86/x64/ARM.
The gc_helper_get_sp function from gchelper_m3.s is not really GC related
and was only used by cc3200, so it has been moved to that port and renamed
to cortex_m3_get_sp.
Add the project file to the mpy-cross directory, which is also where the
executable ends up, and change the Appveyor settings to build mpy-cross
with both msvc and mingw-w64 and verify this all works by running tests
with --via-mpy.
