gas: add new command line options to control diagnostic informational messages

gas: add new command line options to control diagnostic informational messages

gas currently emits informational messages for context information along warnings.
In the context of system register tests in AArch64 backend, these messages
pollute the tests when checking for error message patterns in stderr output.

This patch aims at providing two new flags while preserving the existing
behavior if none of the options is provided.
  * --info, similar to the existing --warn flag to enable diagnostic
    informational messages (default behavior).
  * --no-info, similar to the existing --no-warn flag to disable diagnostic
    informational messages.

It also adds the flags to the existing documentation, and command manual.