gdb, gdbserver, gdbsupport: use [[noreturn]] instead of ATTRIBUTE_NORETURN

C++ 11 has a built-in attribute for this, no need to use a compat macro.

Change-Id: I90e4220d26e8f3949d91761f8a13cd9c37da3875
Reviewed-by: Lancelot Six <lancelot.six@amd.com>