link: expose syscall wrapper symbol helper

[arunsingh]

Fixes #1891

I have exposed a small helper for deriving platform-specific syscall wrapper symbols, then use it from the kprobe_multi syscall attach test.

Kprobe/Kretprobe can retry syscall symbols with the platform prefix, but doing the same automatically for kprobe_multi is ambiguous because a multi-attach list can contain both syscall entrypoints and ordinary kernel functions. The helper lets callers opt in per symbol without changing KprobeMulti attach semantics.

Tests:

• go test ./link -run '^TestSyscallWrapper$' -count=1
• go test -exec sudo ./link -run '^TestKprobeMultiInput$' -count=1
• go test -exec sudo ./link -run '^(TestKprobeMultiProgramCall|TestKprobeMulti|TestKprobeMultiErrors|TestKprobeMultiCookie)$' -count=1 -v
• git diff --check -- link/kprobe.go link/kprobe_test.go link/kprobe_multi_test.go

Also attempted go test -exec sudo ./... in the Ubuntu VM. The focused link tests passed; the broad run hit unrelated cmd/bpf2go module-cache setup with GOPROXY=off and an unrelated features/LircMode2 kernel capability failure.