bpf: selftests: test_loader: Support __btf_path() annotation

This commit adds support for per-prog btf_custom_path. This is necessary
for testing CO-RE relocations on non-vmlinux types using test_loader
infrastructure.

Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Link: https://lore.kernel.org/r/660ea7f2fdbdd5103bc1af87c9fc931f05327926.1702325874.git.dxu@dxuuu.xyz


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>