set_sca_options() macro changed

For now it also sets stack cache size to 256.
added ability to build against RTEMS

Build process is the same, only at the link stage we add special options and
the RTEMS base libraries to link against. This works for simple RTEMS
applications, YMMV.

RTEMS build is enabled by defining TRIPLE to "patmos-unknown-rtems".
RTEMS_LIBPATH needs to point to $ROOT/local/patmos-unknown-rtems/pasim/lib.