gdb: include --enable-targets in 'show configuration' output

gdb: include --enable-targets in 'show configuration' output

Include the value of configuration flag --enable-targets in the output
of GDB command 'show configuration' and also in the output printed for
'gdb --configuration'.  This will make it easier to see how GDB was
built.

No tests added or updated as we can't really check for a specific flag
appearing or not appearing on the configuration output.  But we do
print the configuration within lib/gdb.exp to check which features are
built into GDB, so if this change broke configuration printing then
plenty of tests should stop working (they don't).

Approved-By: Tom Tromey <tom@tromey.com>