libbpf: call dup2() syscall directly

We've ran into issues with using dup2() API in production setting, where
libbpf is linked into large production environment and ends up calling
unintended custom implementations of dup2(). These custom implementations
don't provide atomic FD replacement guarantees of dup2() syscall,
leading to subtle and hard to debug issues.

To prevent this in the future and guarantee that no libc implementation
will do their own custom non-atomic dup2() implementation, call dup2()
syscall directly with syscall(SYS_dup2).

Note that some architectures don't seem to provide dup2 and have dup3
instead. Try to detect and pick best syscall.

Signed-off-by: Andrii Nakryiko <[email protected]>
Acked-by: Song Liu <[email protected]>
Acked-by: Yonghong Song <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

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