C compiler intrinsics can only appear as part of a function call. When called
they are implicitly cast to a function pointer; treat this as a non-null
pointer so that it emits as a regular Zig function call.
Put `pub usingnamespace @import("std").c.builtins;` at the top of translated
C files so that they will have access to builtin functions defined there.
Fixes#6707
