fix some formatting nits
call set_binding_oracle; other minor fixes

This arranges for new_gdb_gcc_instance to call the set_binding_oracle
method.  It adds a cleanup function that just calls
delete_gdb_gcc_instance.  It changes "fe_context" to be local to
get_gcc_jit_context and then changes eval_gcc_jit_command to call
new_gdb_gcc_instance.