gas/testsuite: test-case for PR25331 (mmix reloc fixup bug)

The bug manifested "only" for a 64-bit host:

pr25331.c: Assembler messages:
pr25331.c:430: Error: internal error: fixup not contained within frag
failed with: <pr25331.c: Assembler messages:
pr25331.c:430: Error: internal error: fixup not contained within frag>, no expected output
FAIL: gas/mmix/pr25331

gas:
	PR gas/25331
	* testsuite/gas/mmix/pr25331.d, testsuite/gas/mmix/pr25331.s: New test.