gdb.base/charset.{c,exp} C++ify

gdb.base/charset.{c,exp} C++ify

Adjust gdb.base/charset.{c,exp} so that the testcase works when
compiled as a C++ program.

wchar_t is built-in in C++, so don't make a phony typedef.

The "print /d" is so that we also get "1" instead of "true" in C++
mode.

gdb/testsuite/ChangeLog:

	* gdb.base/charset.c [__cplusplus] (wchar_t, char16_t, char32_t):
	Don't define.
	(utf_32_string): Compile for both C and C++.
	* gdb.base/charset.exp: Use "print /d".