Reserve space in pthread structures for future extensibility.

Make our structures equal in size with glibc structures. This should
give us plenty of space to implement any missing feature.

Bug: 13278744
Bug: 12875898
(cherry picked from commit cf83fd77caa3128ecb6ac935e342cb0ea258b9ee)

Change-Id: I76968d31024eb51bc73887687e5ac492eb02a27f
1 file changed