Fix build errors in with clang in gdb.compile/compile-cplus.c

Fix build errors in with clang in gdb.compile/compile-cplus.c

Clang fails to compile the file, with the following error:
  fatal error: 'iostream' file not found

This prevents the following testcase from executing:
  gdb.compile/compile-cplus.exp

The testcase sets additional_flags when building with GCC, which
this commit causes to also be set when building with clang.  This
makes the testcase fail to build with a different error:
  warning: treating 'c' input as 'c++' when in C++ mode, this behavior
  is deprecated [-Wdeprecated]
so this commit adds -Wno-deprecated in two places to sidestep this.
Note that, while allowing the testcase to build, this commit reveals
failures when the testsuite is built using clang.

gdb/testsuite/ChangeLog:

	* gdb.compile/compile-cplus.exp (additional_flags): Also
	set when building with clang.
	(additional_flags, srcfilesoptions): Pass -Wno-deprecated
	when building with clang.