Clean up TLS_SLOT_BIONIC_PREINIT usage a bit

 - It is only needed for dynamic executables, so move the initialization
   out of __libc_init_main_thread and just before the solib constructor
   calls. For static executables, the slot was initialized, then never
   used or cleared. Instead, leave it clear.

 - For static executables, __libc_init_main_thread already initialized the
   stack guard, so remove the redundant __init_thread_stack_guard call.

 - Simplify the slot access/clearing a bit in __libc_preinit.

 - Remove the "__libc_init_common() will change the TLS area so the old one
   won't be accessible anyway." comment. AFAICT, it's incorrect -- the
   main thread's TLS area in a dynamic executable is initialized to a
   static pthread_internal_t object in the linker, then reused by libc.so.

Test: adb shell /data/nativetest/bionic-unit-tests/bionic-unit-tests
Test: adb shell /data/nativetest/bionic-unit-tests-static/bionic-unit-tests-static
Change-Id: Ie2da6f5be3ad563fa65b38eaadf8ba6ecc6a64b6