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