tests/lint: make test d_gcc_extension platform-independent

That test was intended to test the keywords '__extension__' and
'__typeof'.  The GCC builtin functions were just a side-effect.

These built-in functions generated error messages on platforms such as
amd64 where sizeof(long double) != sizeof(double), but not on others
such as sparc.

The current infrastructure for the lint tests cannot handle tests with
platform-dependent outcome.

(rillig)