Fixes these warnings from go vet:
buildid_linux.go:25: no formatting directive in Fatalf call
callback.go:180: arg pc[i] for printf verb %p of wrong type: uintptr
env.go:34: possible misuse of unsafe.Pointer
issue7665.go:22: possible misuse of unsafe.Pointer

Change-Id: I83811b9c10c617139713a626b4a34ab05564d4fe
Reviewed-on: https://go-review.googlesource.com/15802Reviewed-by: David Crawshaw <crawshaw@golang.org>