gdb/testsuite: add gcore_cmd_available predicate proc

Add a new gcore_cmd_available predicate proc that can be used in a
'requires' line, and make use of it in a few tests.

All of the tests I have modified call gdb_gcore_cmd as one of their
first actions and exit if the gcore command is not available, so it
makes sense (I think) to move the gcore command check into a requires
call.

There should be no change in what is actually run after this commit.