fix: restore absolute DWARF source paths

[immanuwell]

Clang 22 can emit an absolute DWARF file name alongside a compilation directory
Go's DWARF reader joins them, so stackwhere reports /go/src/basic.c even though the real path is /src/basic.c

Repro:

audit_dir=$(mktemp -d)
docker run -v "$PWD/testdata:/src:ro" -v "$audit_dir:/out" -w /go ghcr.io/cilium/ebpf-builder:1777990914 clang-22 -g -O2 -target bpf -c /src/basic.c -o /out/basic.o
go run ./cmd/stackwhere list "$audit_dir/basic.o" cil_entry

Before: 32 - a @ /go/src/basic.c:72

After: 32 - a @ /src/basic.c:72

The fix restores the compilation unit path only when the bad joined form matches exactly
Relative and already-correct paths stay as-is, regression coverage is included too

Tests: go test ./..., go test -race ./..., go vet ./..., golangci-lint run

[dylandreimerink]

🙏