gdb/disasm.h: Mark callbacks noexcept unconditionally

Given that C+17 is now a requirement for GBD, update gdb/disasm.h to
define callback function types noexcept unconditionally.  The pre-C++17
configuration is not supported anymore.

Change-Id: I0a38e22b7912c70a11425363a991f0b01614343e
1 file changed