[BPF] Introduce support for type match relocations

Among others, BPF currently supports the type-exists CO-RE relocation
(e.g., see D83878 & D83242). Its intention, as the name tries to convey,
is to be used for checking existence of a type in a target.
While that check is useful and has its place, we would also like to
be able to perform stricter type queries: instead of just checking mere
existence, we want to make sure that members match up in composite
types, that enum variants are present, etc. We refer to this as "type
match".

This change proposes the addition of a new relocation variant/value that
we intend to use for establishing this match relation.

Differential Revision: https://reviews.llvm.org/D126838