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>
1 file changed