rust: Emit -x c++ for bindgen modules with cpp_std

rust_bindgen modules which defined cpp_std with a .h file were not
correctly emitting the `-x c++` flag. This CL addresses that, and
ensures that if either cpp_std or c_std is set then the appropriate
behavior occurs no matter the file extension.

Bug: 304269101
Test: Soong tests
Change-Id: I71a8ae30ac0ed502d9d3fbf2f3039b0c56529d39