cli: Fix parsing of --export flags

Those flags, even when passed multiple times, are always prepend with
`--export` when they come from rustc. Delimiter can be used, but we
shouldn't include any positional arguments after the flag as a part of
that flag. Therefore, we use the combination of
`use_value_delimiter = true` and `num_args = 1` options to ensure that.

That bug can be reproduced by running bpf-linker together with rustc
after rust-lang/rust@7f06d51
which passes the following sequence of arguments to the linker:

```
[...]
--export
connect
--export
some_dep
/tmp/rusdep
/tmp/rustcwUehHK/symbols.o
/tmp/bin.bin.3b77bbea-cgu.0.rcgu.o
-L
/tmp
-L
/home/vadorovsky/repos/bpf-linker/target/debug/deps
[...]
```

`/tmp/rustdep`, `symbols.o` and `*.rcgu.o` are inputs, but
bpf-linker was parsing them as symbols to export. Therefore, `symbols.o`
and `*.rcgu.o` were not linked, bpf-linker was ending up just linking
dependencies / external crates together and not including the main
program.

Fixes: aya-rs#27
Signed-off-by: Michal Rostecki <vadorovsky@gmail.com>

Author: vadorovsky

Cargo.lock

@@ -12,7 +12,7 @@ edition = "2021"
 
 [dependencies]
 # cli deps
-clap = { version = "3.2", optional = true, features = ["derive"] }
+clap = { version = "4.0", optional = true, features = ["derive"] }
 simplelog = {version = "0.7.6", optional = true}
 
 # lib deps