selftests/bpf: tests for jmp to 1st insn

Add 2 tests that check JIT code generation to jumps to 1st insn.
1st test is similar to syzbot reproducer.
The backwards branch is never taken at runtime.
2nd test has branch to 1st insn that executes.
The test is written as two bpf functions, since it's not possible
to construct valid single bpf program that jumps to 1st insn.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>