cgo: add dummy implementation of __builtin_va_list

Every ABI has a slightly different implementation. Ideally, we would use
something like Clang TargetInfo or extract it by compiling some C code
and checking the IR, but this is a useful workaround for now.

loader/cgo.go

@@ -60,6 +60,7 @@ var cgoAliases = map[string]string{
 	"C.uint32_t":          "uint32",
 	"C.uint64_t":          "uint64",
 	"C.uintptr_t":         "uintptr",
+	"C.__builtin_va_list": "uintptr", // dummy value until fully implemented
 }
 
 // cgoTypes lists some C types with ambiguous sizes that must be retrieved