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

Fixes #27508, #17343

Change-Id: I3e50ae781012a093254f7f8d6d719d89426f04a9
GitHub-Last-Rev: 7c3232d8eeb76cb79e8b0a232ccf80569c1e50de
GitHub-Pull-Request: golang/mobile#20
Reviewed-on: https://go-review.googlesource.com/134975
Reviewed-by: Elias Naur <elias.naur@gmail.com>
diff --git a/cmd/gomobile/env.go b/cmd/gomobile/env.go
index 8714dbb..a5dc19e 100644
--- a/cmd/gomobile/env.go
+++ b/cmd/gomobile/env.go
@@ -127,8 +127,9 @@
 			"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",
 		)