gdb/disasm.h: Mark callbacks noexcept unconditionally Given that C++17 is now a requirement for GDB, update gdb/disasm.h to define callback function types noexcept unconditionally. The pre-C++17 configuration is not supported anymore. Change-Id: I0a38e22b7912c70a11425363a991f0b01614343e Approved-By: Tom Tromey <tom@tromey.com> Approved-By: Pedro Alves <pedro@palves.net>