Handle the GCC restriction like the set list by explicitly checking for
HAVE_GCC. When using EXTERNAL_TOOLCHAIN with LLVM, it would still be
picked up otherwise.

Test case requires the GCC specific thread extension for Objective C, so
only enable the test if GCC is present and disable other compiler.

Convert the libobjc tests to atf. While doing this, make the tests actually
check that things work.