btf: fix CO-RE bitfield relocations

[lmb]

program: make CO-RE relocation failures more descriptive

It's difficult to figure out what went wrong during CO-RE relocation since
we don't know what the original relocation was. Add it to the error message:

    apply CO-RE relocations: fixup for CORERelocation(signed,
   Struct:"bits"[0:5], local_id=6): invalid immediate 0, expected 1
   (fixup: signed=1->1, kind: signed, ins: MovImm dst: r2 imm: 0)

Signed-off-by: Lorenz Bauer <lmb@isovalent.com>

btf: fix reloFieldSigned

reloFieldSigned is used to figure out whether a struct or union field is
signed or unsigned. As such it must use coreAccessor to traverse a type and
find the correct fields. This currently doesn't happen and so we always
calculate the fixup against the struct or union. This means we never
classify a field as signed.

This behaviour has masked another bug: the current code assumes that enums
are always signed. Fix this by deriving the correct value from Enum.Signed.

Fix reloFieldSigned by moving the logic in with the other field based
relocation logic. This also fixes a possible panic due to an incorrect type
assertion when local and target are not of the same type.

Signed-off-by: Lorenz Bauer <lmb@isovalent.com>

btf: return an error when CO-RE bitfield relo encounters unsized type

It is valid to access a non-bitfield member of a struct using bitfield 
relocations. This happens when a member goes from being a bitfield to a
regular field. In this case we synthesize the size of the bitfield shifts
via the size of the regular field. Due to a bug we don't return an error
when retrieving the size.

Fix this by bubbling up the error and adding some testcases.

Signed-off-by: Lorenz Bauer <lmb@isovalent.com>

btf: return an error if type has no signedness

Refuse to create a fixup for reloFieldSigned if the type isn't an Int or an
Enum. On clang-14 it's not even possible to create such a relocation, so no
need to be extra lenient.

Add a unit test since we can't get clang to emit a wonky relocation.

Signed-off-by: Lorenz Bauer <lmb@isovalent.com>