selftests/bpf: Test TYPE_EXISTS and TYPE_SIZE CO-RE relocations

Add selftests for TYPE_EXISTS and TYPE_SIZE relocations, testing correctness
of relocations and handling of type compatiblity/incompatibility.

If __builtin_preserve_type_info() is not supported by compiler, skip tests.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20200819194519.3375898-3-andriin@fb.com