pthread_key.cpp: factor out get_thread_key_data

This change makes it easier to move the location of the pthread keys
(e.g. into the bionic_tls struct).

Bug: http://b/78026329
Test: bionic unit tests
Test: disassembly of libc.so doesn't change
Change-Id: Ib75d9dab8726de96856af91ec3daa2c5cdbc2178