runtime/cgo: only build _cgo_callers if x_cgo_callers is defined
Fixes a problem when using the external linker on Solaris. The Solaris
external linker still doesn't work due to issue #14957.
The problem is, for example, with `go test cmd/objdump`:
objdump_test.go:71: go build fmthello.go: exit status 2
# command-line-arguments
/var/gcc/iant/go/pkg/tool/solaris_amd64/link: running gcc failed: exit status 1
Undefined first referenced
symbol in file
x_cgo_callers /tmp/go-link-
355600608/go.o
ld: fatal: symbol referencing errors
collect2: error: ld returned 1 exit status
Change-Id: I54917cfd5c288ee77ea25c439489bd2c9124fe73
Reviewed-on: https://go-review.googlesource.com/21392
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>