gdb: Pass -x c++ to the compiler

gdb: Pass -x c++ to the compiler

Because we are compiling .c files containing C++ code, clang++ complains
with:

  clang: error: treating 'c' input as 'c++' when in C++ mode, this behavior is deprecated

If renaming all the source files to .cpp is out of the question, an
alternative is to pass "-x c++" to convince the compiler that we are
really compiling C++.  It works fine with GCC too.

gdb/ChangeLog:

	* Makefile.in (COMPILE.pre): Add "-x c++".

gdb/gdbserver/ChangeLog:

	* Makefile.in (COMPILE.pre): Add "-x c++".