linker: Make linker_{error,warning} generally available

linker: Make linker_{error,warning} generally available

linker_warning is a new function.  It's identical to linker_error
except that it doesn't set LinkStatus=false and it prepends "warning: "
on messages instead of "error: ".

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>