[C++] Switch TRY/CATCH to real C++ try/catch by default again

Now that we don't ever throw GDB exceptions from signal handlers [1],
we can switch back to having TRY/CATCH implemented in terms of C++
try/catch instead of sigjmp/longjmp.

[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.h (GDB_XCPT_TRY): Update comment.
	[__cplusplus] (GDB_XCPT): Define as GDB_XCPT_TRY.

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2016-04-12  Pedro Alves  <palves@redhat.com>
+
+	* common/common-exceptions.h (GDB_XCPT_TRY): Update comment.
+	[__cplusplus] (GDB_XCPT): Define as GDB_XCPT_TRY.
+
 2016-04-12  Pedro Alves  <palves@redhat.com>
 
 	* common/common-exceptions.c (struct catcher) <buf>: Now a

gdb/common/common-exceptions.h

@@ -122,9 +122,7 @@ struct gdb_exception
    the only mode supported when GDB is built as a C program.  */
 #define GDB_XCPT_SJMP 1
 
-/* Make GDB exceptions use try/catch behind the scenes.  Can't be made
-   the default until we stop throwing exceptions from signal
-   handlers.  */
+/* Make GDB exceptions use try/catch behind the scenes.  */
 #define GDB_XCPT_TRY 2
 
 /* Specify this mode to build with TRY/CATCH mapped directly to raw
@@ -133,8 +131,11 @@ struct gdb_exception
    spurious code between the TRY and the CATCH block.  */
 #define GDB_XCPT_RAW_TRY 3
 
-/* Always use setjmp/longmp, even in C++ mode.  */
-#define GDB_XCPT GDB_XCPT_SJMP
+#ifdef __cplusplus
+# define GDB_XCPT GDB_XCPT_TRY
+#else
+# define GDB_XCPT GDB_XCPT_SJMP
+#endif
 
 /* Functions to drive the exceptions state machine.  Though declared
    here by necessity, these functions should be considered internal to