selftests/bpf: Add unit tests for pointers in global functions

test_global_func9  - check valid pointer's scenarios
test_global_func10 - check that a smaller type cannot be passed as a
                     larger one
test_global_func11 - check that CTX pointer cannot be passed
test_global_func12 - check access to a null pointer
test_global_func13 - check access to an arbitrary pointer value
test_global_func14 - check that an opaque pointer cannot be passed
test_global_func15 - check that a variable has an unknown value after
		     it was passed to a global function by pointer
test_global_func16 - check access to uninitialized stack memory

test_global_func_args - check read and write operations through a pointer

Signed-off-by: Dmitrii Banshchikov <me@ubique.spb.ru>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210212205642.620788-5-me@ubique.spb.ru