Add extra compile options for mission scope and arch scope.

These are separated to support cross compile environments that
do not/cannot use the same flags on both builds.

For "mission" build the targets are never cross compiled, only
built for the native host machine.  It should be safe to assume
a compiler in the GCC family and the strict warnings should
_always_ be applicable here.

For "arch" build the options are compiler vendor dependent.  The
file as-supplied can only be used if all the target cross compilers
are in the same family and support the same warning options.
However, this file can be modified without affecting the options
used for the host side tools.

Add a _custom suffix to differentiate the customization
file from the base file in the cmake directory.

Include in the basic warning set.  Note that at this time the CFE
does not build cleanly on all architectures with this warning enabled,
pending resolution of issue nasa#313.