crypto/x509: darwin-386 build is broken

[bradfitz]

https://go-review.googlesource.com/c/go/+/162860 broke the darwin-386 builder:

https://build.golang.org/log/d1a0dbe3428c41dfefc1a4d7b4d50735198fe20b

Building packages and commands for darwin/386.
# crypto/x509
src/crypto/x509/root_cgo_darwin.go:218:55: warning: values of type 'SInt32' should not be used as format arguments; add an explicit cast to 'int' instead [-Wformat]
go build crypto/x509: C compiler warning promoted to error on Go builders
go tool dist: FAILED: /private/var/folders/dx/k53rs1s93538b4x20g46cj_w0000gn/T/workdir/go/pkg/tool/darwin_386/go_bootstrap install -gcflags=all= -ldflags=all= std cmd: exit status 1

/cc @FiloSottile @dmitshur @ianlancetaylor

[gopherbot]

Change https://golang.org/cl/164240 mentions this issue: [release-branch.go1.11] crypto/x509: explicitly cast printf format argument

[gopherbot]

Closed by merging 8390781 to release-branch.go1.11.