No. We might as well as totally turn builtins off since the goal is to
have the whole tree under WARNS=2. Since you want to change
functionality, the burdon is on you to convince us why you want to do this
-- which you have not.
The alternatives - just to get it to build - are to turn off -Wshadow,
or rename every single local variable in the tree that shadows a
builtin (and believe me, there are *tons* of these).
In addition, there's the issue of errors that -fno-builtin hides -
many of those have been fixed already, but I don't know how many
remain (or how many will be introduced in the future).