[Libomptarget] Don't use full names for exported plugin symbols

Summary:
This patch changes the `exports` file to export all `__tgt_rtl`
functions. This is a better option as not each plugin implements all of
these functions, furthermore any new functions added will be
automatically included.

openmp/libomptarget/plugins/exports

@@ -1,40 +1,6 @@
 VERS1.0 {
   global:
-    __tgt_rtl_init_plugin;
-    __tgt_rtl_deinit_plugin;
-    __tgt_rtl_is_valid_binary;
-    __tgt_rtl_is_valid_binary_info;
-    __tgt_rtl_is_data_exchangable;
-    __tgt_rtl_number_of_devices;
-    __tgt_rtl_init_requires;
-    __tgt_rtl_init_device;
-    __tgt_rtl_deinit_device;
-    __tgt_rtl_load_binary;
-    __tgt_rtl_data_alloc;
-    __tgt_rtl_data_submit;
-    __tgt_rtl_data_submit_async;
-    __tgt_rtl_data_retrieve;
-    __tgt_rtl_data_retrieve_async;
-    __tgt_rtl_data_exchange;
-    __tgt_rtl_data_exchange_async;
-    __tgt_rtl_data_delete;
-    __tgt_rtl_run_target_team_region;
-    __tgt_rtl_run_target_team_region_async;
-    __tgt_rtl_run_target_region;
-    __tgt_rtl_run_target_region_async;
-    __tgt_rtl_synchronize;
-    __tgt_rtl_register_lib;
-    __tgt_rtl_unregister_lib;
-    __tgt_rtl_supports_empty_images;
-    __tgt_rtl_set_info_flag;
-    __tgt_rtl_print_device_info;
-    __tgt_rtl_create_event;
-    __tgt_rtl_record_event;
-    __tgt_rtl_wait_event;
-    __tgt_rtl_sync_event;
-    __tgt_rtl_destroy_event;
-    __tgt_rtl_init_device_info;
-    __tgt_rtl_init_async_info;
+    __tgt_rtl*
   local:
     *;
 };