[AArch64][committed] Fix gcc.target/aarch64/spellcheck_1.c and spellcheck_4.c

[AArch64][committed] Fix gcc.target/aarch64/spellcheck_1.c and spellcheck_4.c

These two tests started failing after commit r264335 that adjusted the cutoff point at which the diagnostic suggestions machinery decides a suggestion is meaningful.
For these tests it means we no longer suggest anything as an alternative to "armv8-a-typo" as an "arch=" pargma value. We do still list the valid options, we just don't prefer one particular value over the others.

When I first wrote this test it wasn't with a particular architecture suggestion in mind, but rather to test that the suggestion machinery is being sanely invoked.
So this patch changes the dg-message check to treat the "did you mean...?" hunk as optional (in case the heuristics in the suggestions machinery change again).

With this patch the two tests PASS again on aarch64.

    * gcc.target/aarch64/spellcheck_1.c:
    Make architecture suggestion optional.
    * gcc.target/aarch64/spellcheck_4.c:
    Likewise.

From-SVN: r264390