eco32: implement RETURN_ADDR_RTX

This is required for __builtin_return_address () to work.
Only the minimum requirement of support for the current frame is
fulfilled with this implementation.
eco32: fix softfp library configuration

None of the softfp library code was linked in to libgcc.
Remove everything in the t- files and add t-softfp-sfdf t-softfp
to tmake_file to fix it.