Use setjmp/longjmp for TRY/CATCH instead of sigsetjmp/siglongjmp

Now that we don't ever throw GDB exceptions from signal handlers [1],
we can switch to have TRY/CATCH implemented in terms of plain
setjmp/longjmp instead of sigsetjmp/siglongjmp.

In https://sourceware.org/ml/gdb-patches/2015-02/msg00114.html, Yichun
Zhang mentions a 11%/14%+ speedup in his GDB python scripts with a
patch that did something similar to only a specific set of TRY/CATCH
calls.

[1] - https://sourceware.org/ml/gdb-patches/2016-03/msg00351.html

Tested on x86_64 Fedora 23, native and gdbserver.

gdb/ChangeLog:
2016-04-12  Pedro Alves  <palves@redhat.com>

	* common/common-exceptions.c (struct catcher) <buf>: Now a
	'jmp_buf' instead of SIGJMP_BUF.
	(exceptions_state_mc_init): Change return type to 'jmp_buf'.
	(throw_exception): Use longjmp instead of SIGLONGJMP.
	* common/common-exceptions.h: Include <setjmp.h> instead of
	"gdb_setjmp.h".
	(exceptions_state_mc_init): Change return type to 'jmp_buf'.
	[GDB_XCPT == GDB_XCPT_SJMP] (TRY): Use setjmp instead of
	SIGSETJMP.
	* cp-support.c: Include "gdb_setjmp.h".

gdb/cp-support.c

@@ -34,7 +34,7 @@
 #include "cp-abi.h"
 #include "namespace.h"
 #include <signal.h>
-
+#include "gdb_setjmp.h"
 #include "safe-ctype.h"
 
 #define d_left(dc) (dc)->u.s_binary.left