libbpf: Add uprobe ref counter offset support for USDT semaphores

When attaching to uprobes through perf subsystem, it's possible to specify
offset of a so-called USDT semaphore, which is just a reference counted u16,
used by kernel to keep track of how many tracers are attached to a given
location. Support for this feature was added in [0], so just wire this through
uprobe_opts. This is important to enable implementing USDT attachment and
tracing through libbpf's bpf_program__attach_uprobe_opts() API.

  [0] a6ca88b2 ("trace_uprobe: support reference counter in fd-based uprobe")

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210815070609.987780-16-andrii@kernel.org