gdb/Makefile: Print 'GEN' message, and pass SILENT_FLAG more

The targets that use config.status to regenerate themselves don't
currently follow the silent rules that the rest of GDB's Makefile
does.  For example, touch the gdb/gcore.in file and then 'make all' in
the gdb/ directory prints:

  /bin/sh config.status gcore
  config.status: creating gcore

In this commit I make use of the silent-rules.mk mechanism for these
targets, now we get:

  GEN    gcore

Which matches the rest of our Makefile.  Obviously, if you pass 'V=1'
to the build then you'll get the old output back.

There's no change in what is generated after this commit.

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