cmd/cgo/internal/test: don't pass -lm on darwin

On darwin, -lm is not necessary as the math functions are included
in libSystem. Passing -lm multiple times results in linker
warnings. Don't pass it on darwin.

For #61229.

Change-Id: I72d8dab1f0eead68cbeb176ac97b8ed1a0cfddab
Reviewed-on: https://go-review.googlesource.com/c/go/+/508697
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>

src/cmd/cgo/internal/test/issue8756.go

@@ -1,7 +1,7 @@
 package cgotest
 
 /*
-#cgo LDFLAGS: -lm
+#cgo !darwin LDFLAGS: -lm
 #include <math.h>
 */
 import "C"

src/cmd/cgo/internal/test/issue8756/issue8756.go

@@ -1,7 +1,7 @@
 package issue8756
 
 /*
-#cgo LDFLAGS: -lm
+#cgo !darwin LDFLAGS: -lm
 #include <math.h>
 */
 import "C"

src/cmd/cgo/internal/test/test.go

@@ -23,7 +23,7 @@ package cgotest
 #include <unistd.h>
 #include <sys/stat.h>
 #include <errno.h>
-#cgo LDFLAGS: -lm
+#cgo !darwin LDFLAGS: -lm
 
 #ifndef WIN32
 #include <pthread.h>