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

[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.