gdb/Makefile: add some missing config.status dependencies

gdb/Makefile: add some missing config.status dependencies

I noticed that for the build targets jit-reader.h, gcore, gdb-gdb.py,
and gdb-gdb.gdb the rules all use the config.status script, but don't
have a dependency on the config.status target.  This means we might
fail to regenerate these targets in a case where config.status, or one
of its dependencies changes.

Two other targets that use config.status do correctly have a
dependency on config.status.

Fixed in this commit by adding the missing dependencies.

There should be no changes in _what_ is generated after this commit.

Approved-By: Simon Marchi <simon.marchi@efficios.com>