gdb: add callback defines for new ARGV handling

gdb: add callback defines for new ARGV handling

The common sim code has slightly unfinished support for these already,
but even arch ports are unable to handle these if the common header does
not define them.  This is because the generated callback header includes
simple common gdb/sim headers only which causes it to skip the new ARGV
syscalls.  Plus, it isn't like providing these in the common header will
break any sim targets which don't want them.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>