E3-921: Add test to ensure that failed architectures cause build to fail

This adds a test that checks that if a single architecture fails, then the build
fails as a whole.

There are some slight complications in terms of writing good tests, since we do not
want to use real architectures. Since the source files are converted from
`SOURCES -> SRCS -> LIBOBJS` and we finally have the dependency
```make
${DEPFILE}: ${LIBOBJS}
```
we need to ensure that `LIBOBJS` is properly populated before we get to this point.
The challenge, however, is that `LIBOBJS` is defined (in terms of `SRCS` as
```make
LIBOBJS += $(addsuffix $(OBJ),$(notdir $(basename $(filter-out %.$(OBJ) %$(LIB_SUFFIX),$(sort ${SRCS})))))
```
and `LIB_SUFFIX` is set in `${CONFIG}/os/CONFIG.Common.${T_A}`. This is why
we need to do the workaround of hand-defining it at the beginning.