selftests/bpf: Add test for narrow spill into 64-bit spilled scalar

Add a test case to verify that without CAP_PERFMON, the test now
succeeds instead of failing due to a verification error.

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20241204044757.1483141-6-memxor@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>