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Since e19d702b, dlsym and friends use recursive mutexes that
require the current thread id, which is not available before the libc
constructor. This prevents us from using dlsym() in .preinit_array.

This change moves TLS initialization from libc constructor to the earliest
possible point - immediately after linker itself is relocated. As a result,
pthread_internal_t for the initial thread is available from the start.

As a bonus, values stored in TLS in .preinit_array are not lost when libc is
initialized.

Change-Id: Iee5a710ee000173bff63e924adeb4a4c600c1e2d