Split main thread init into early+late functions

Split __libc_init_main_thread into __libc_init_main_thread_early and
__libc_init_main_thread_late. The early function is called very early in
the startup of the dynamic linker and static executables. It initializes
the global auxv pointer and enough TLS memory to do system calls, access
errno, and run -fstack-protector code (but with a zero cookie because the
code for generating a cookie is complex).

After the linker is relocated, __libc_init_main_thread_late finishes
thread initialization.

Bug: none
Test: bionic unit tests
Change-Id: I6fcd8d7587a380f8bd649c817b40a3a6cc1d2ee0