selftests/bpf: verifier/bpf_get_stack converted to inline assembly

Test verifier/bpf_get_stack automatically converted to use inline assembly.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20230421174234.2391278-4-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>