doc: Ignore GNU attribute syntax in Doxygen

Without defining out GNU attributes, Doxygen will fail to interpret them
and bail out, causing CI errors. This commit enables macro expansion for
a list of predefined macros and adds a macro to hide GNU attributes.

Change-Id: Id155a0cc75ec4ad10c0d581b28cb739b79dd0f80
Signed-off-by: Chris Kay <chris.kay@arm.com>