floss: Add cxxcall and mutcxxcall macros

Add new macros for calling cxx methods. Use of these macros will
standardize how we store and use these methods (making it easier to
replace or protect with guards in the future).

Bug: 189497489
Tag: #floss
Test: emerge-zork floss
Change-Id: I1ce49aa51f516eb8004da8a83b74a8d992adecf0