testsuite: Increase width of stack003 test
testsuite: Increase width of stack003 test

Previously the returned tuple seemed to fit in registers on amd64. This
meant that non-moving collector bug would cause the test to fail on i386
yet not amd64.

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc