configure: Add probe identification of MS armasm

Signed-off-by: Martin Storsjö <martin@martin.st>
configure: Enable gas-preprocessor on all OSes but only if available

Signed-off-by: Martin Storsjö <martin@martin.st>
configure: Suggest upgrading gas-preprocessor instead of just installing

Older versions don't support all the features we test for now.

Signed-off-by: Martin Storsjö <martin@martin.st>