* lib/gas-defs.exp (run_dump_test): Support new attribute "stderr"
which is a file used to regexp_diff the assembler's pruned stderr
output.
(write_file): New helper function to write a string into a file.
* gas/mips/empic.l: New file which matches empic tests' warnings.
* gas/mips/empic.d: Use empic.l.