Add RBE_CXX_COMPARE variable to locally disable compare mode for C++

When we do 64-32 migration, we want compare mode to be present only for
non-c++ actions as it would impact performance on 32-core machines
otherwise.

Bug: b/157414300
Test: Ran a build with "RBE_CXX_LINKS="true" RBE_CXX_LINKS_EXEC_STRATEGY="remote" RBE_CXX_COMPARE="false" RBE_compare="true" use_rbe mmma bionic"
and observed that local-executions happened only for link actions and
not for C++ actions.

Change-Id: Ia972c74d804085196a7b6ec755af63042c6b3909
diff --git a/core/rbe.mk b/core/rbe.mk
index 019ac04..9ec8481 100644
--- a/core/rbe.mk
+++ b/core/rbe.mk
@@ -28,6 +28,12 @@
     cxx_rbe_exec_strategy := "local"
   endif
 
+  ifdef RBE_CXX_COMPARE
+    cxx_compare := $(RBE_CXX_COMPARE)
+  else
+    cxx_compare := "false"
+  endif
+
   ifdef RBE_JAVAC_EXEC_STRATEGY
     javac_exec_strategy := $(RBE_JAVAC_EXEC_STRATEGY)
   else
@@ -51,7 +57,7 @@
   java_r8_d8_platform := $(platform)",Pool=java16"
 
   RBE_WRAPPER := $(rbe_dir)/rewrapper
-  RBE_CXX := --labels=type=compile,lang=cpp,compiler=clang --env_var_whitelist=PWD --exec_strategy=$(cxx_rbe_exec_strategy) --platform="$(cxx_platform)"
+  RBE_CXX := --labels=type=compile,lang=cpp,compiler=clang --env_var_whitelist=PWD --exec_strategy=$(cxx_rbe_exec_strategy) --platform="$(cxx_platform)" --compare="$(cxx_compare)"
 
   # Append rewrapper to existing *_WRAPPER variables so it's possible to
   # use both ccache and rewrapper.