bpf, libbpf: Add type match support

This patch adds support for the proposed type match relation to
relo_core where it is shared between userspace and kernel. It plumbs
through both kernel-side and libbpf-side support.

The matching relation is defined as follows (copy from source):
- modifiers and typedefs are stripped (and, hence, effectively ignored)
- generally speaking types need to be of same kind (struct vs. struct, union
  vs. union, etc.)
  - exceptions are struct/union behind a pointer which could also match a
    forward declaration of a struct or union, respectively, and enum vs.
    enum64 (see below)
Then, depending on type:
- integers:
  - match if size and signedness match
- arrays & pointers:
  - target types are recursively matched
- structs & unions:
  - local members need to exist in target with the same name
  - for each member we recursively check match unless it is already behind a
    pointer, in which case we only check matching names and compatible kind
- enums:
  - local variants have to have a match in target by symbolic name (but not
    numeric value)
  - size has to match (but enum may match enum64 and vice versa)
- function pointers:
  - number and position of arguments in local type has to match target
  - for each argument and the return value we recursively check match

Signed-off-by: Daniel Müller <deso@posteo.net>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220628160127.607834-5-deso@posteo.net

src/relo_core.h

@@ -72,6 +72,10 @@ int __bpf_core_types_are_compat(const struct btf *local_btf, __u32 local_id,
 				const struct btf *targ_btf, __u32 targ_id, int level);
 int bpf_core_types_are_compat(const struct btf *local_btf, __u32 local_id,
 			      const struct btf *targ_btf, __u32 targ_id);
+int __bpf_core_types_match(const struct btf *local_btf, __u32 local_id, const struct btf *targ_btf,
+			   __u32 targ_id, bool behind_ptr, int level);
+int bpf_core_types_match(const struct btf *local_btf, __u32 local_id, const struct btf *targ_btf,
+			 __u32 targ_id);
 
 size_t bpf_core_essential_name_len(const char *name);