RFA: Add PchIgnore option property

[Joern Rennecke <joern dot rennecke at embecosm dot com>]

I've been looking how to make the precompiled header mechanism allow
me to use the
ARC -misize option (which outputs additional information about gcc's
idea of instruction
addresses for the purpose of branch shortening, to help debugging the
latter) in a
compilation involving precompiled headers.
I can't use TARGET_CHECK_PCH_TARGET_FLAGS for that purpose because
-misize uses its own variable (to save on target_flags bits).
If I wanted to use the TARGET_PCH_VALID_P hook, I'd have to duplicate
lots of code
from default_pch_valid_p, which is intricately tied with the pch
implementation, and
also 'knows' that non-target flags never affect pch.
Moreover, having this extra information encoded in a separate
function, instead of
at the specific option(s) in config/<target>/<target>.opt is rather messy.

Therefore, I propose to add a new option property to mark an option that should
be ignored for the purpose of checking pch validity.

The attached patch implements this as PchIgnore.

bootstrapped on i686-pc-linux.gnu.

OK to apply?

Attachment: pch-ignore-patch
Description: Binary data