selftests/bpf: Support struct/union presets in veristat

Extend commit e3c9abd0 ("selftests/bpf: Implement setting global
variables in veristat") to support applying presets to members of
the global structs or unions in veristat.
For example:
```
./veristat set_global_vars.bpf.o  -G "union1.struct3.var_u8_h = 0xBB"
```

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250408104544.140317-1-mykyta.yatsenko5@gmail.com