bug: support bpf_core_type_exists()

In the previous version, when no target is found in BTF, coreCalculateFixups()
will mark its relocation as poison and cause failure when verified by Linux verifier.

This issue is fixed by using right fixup for "checksForExistence" type of relocations.

Signed-off-by: Shang-Wen Wang (Sam Wang) <sam_s_wang@trendmicro.com>

internal/btf/core.go

@@ -313,10 +313,16 @@ func coreCalculateFixups(byteOrder binary.ByteOrder, local Type, targets []Type,
 
 	if bestFixups == nil {
 		// Nothing at all matched, probably because there are no suitable
-		// targets at all. Poison everything!
+		// targets at all.
+		//
+		// Poison everything except checksForExistence.
 		bestFixups = make([]COREFixup, len(relos))
 		for i, relo := range relos {
-			bestFixups[i] = COREFixup{kind: relo.kind, poison: true}
+			if relo.kind.checksForExistence() {
+				bestFixups[i] = COREFixup{kind: relo.kind, local: 1, target: 0}
+			} else {
+				bestFixups[i] = COREFixup{kind: relo.kind, poison: true}
+			}
 		}
 	}