[C++] Always use setjmp/longjmp for exceptions

[C++] Always use setjmp/longjmp for exceptions

We currently throw exceptions from signal handlers (e.g., for
Quit/ctrl-c).  But throwing C++ exceptions from signal handlers is
undefined.  (That doesn't restore signal masks, like siglongjmp does,
and, because asynchronous signals can arrive at any instruction, we'd
have to build _everything_ with -fasync-unwind-tables to make it
reliable.)  It happens to work on x86_64 GNU/Linux at least, but it's
likely broken on other ports.

Until we stop throwing from signal handlers, use setjmp/longjmp based
exceptions in C++ mode as well.

gdb/ChangeLog:
2015-11-17  Pedro Alves  <palves@redhat.com>

	* common/common-exceptions.h (GDB_XCPT_SJMP, GDB_XCPT_TRY)
	(GDB_XCPT_RAW_TRY, GDB_XCPT): Define.
	Replace __cplusplus checks with GDB_XCPT checks throughout.
	* common/common-exceptions.c: Replace __cplusplus checks with
	GDB_XCPT checks throughout.