cmd/gomobile: specify clang++ compiler for C++ compilation, setup CGO_CXXFLAGS

Fixes #27508, #17343

Change-Id: I3e50ae781012a093254f7f8d6d719d89426f04a9
GitHub-Last-Rev: 7c3232d
GitHub-Pull-Request: #20
Reviewed-on: https://go-review.googlesource.com/134975
Reviewed-by: Elias Naur <[email protected]>

Author: tadovas

cmd/gomobile/env.go

@@ -127,8 +127,9 @@ func envInit() (err error) {
 			"GOOS=darwin",
 			"GOARCH="+arch,
 			"CC="+clang,
-			"CXX="+clang,
+			"CXX="+clang+"++",
 			"CGO_CFLAGS="+cflags+" -arch "+archClang(arch),
+			"CGO_CXXFLAGS="+cflags+" -arch "+archClang(arch),
 			"CGO_LDFLAGS="+cflags+" -arch "+archClang(arch),
 			"CGO_ENABLED=1",
 		)