tombstoned: allow linking tombstones.

Bug: http://b/77729983
Test: debuggerd_test
Test: adb shell 'for x in `seq 0 50`; do crasher; done'
Change-Id: I1d86d04047240a85b2e987116efd9be59607b766

public/tombstoned.te

@@ -9,7 +9,7 @@ allow tombstoned domain:fifo_file write;
 allow tombstoned domain:dir r_dir_perms;
 allow tombstoned domain:file r_file_perms;
 allow tombstoned tombstone_data_file:dir rw_dir_perms;
-allow tombstoned tombstone_data_file:file create_file_perms;
+allow tombstoned tombstone_data_file:file { create_file_perms link };
 
 # TODO: Remove append / write permissions. They were temporarily
 # granted due to a bug which appears to have been fixed.
@@ -19,4 +19,4 @@ auditallow tombstoned anr_data_file:file { append write };
 # Changes for the new stack dumping mechanism. Each trace goes into a
 # separate file, and these files are managed by tombstoned.
 allow tombstoned anr_data_file:dir rw_dir_perms;
-allow tombstoned anr_data_file:file { create getattr open unlink };
+allow tombstoned anr_data_file:file { create getattr open link unlink };