selftests/bpf: Add CFLAGS per source file and runner

selftests/bpf: Add CFLAGS per source file and runner

[ Upstream commit 207cf6e649ee551ab3bdb1cfe1b2848e6a4337a5 ]

This patch adds support to specify CFLAGS per source file and per test
runner.

Signed-off-by: Cupertino Miranda <cupertino.miranda@oracle.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/bpf/20240507122220.207820-2-cupertino.miranda@oracle.com
Stable-dep-of: 3ece93a4087b ("selftests/bpf: Fix wrong binary in Makefile log output")
Signed-off-by: Sasha Levin <sashal@kernel.org>