cmd/compile: use absolute file name in isCgo check
For #23672
Fixes #63211
Fixes CVE-2023-39323
Change-Id: I4586a69e1b2560036afec29d53e53cf25e6c7352
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/
2032884
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Roland Shoemaker <bracewell@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/534158
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>