Move Bionic slots to the end of pthread_internal_t

With ELF TLS, the static linker assumes that an executable's TLS segment
is at a known offset from the thread pointer (i.e. __get_tls()). The
segment can be located prior to the TP (variant 2, x86[_64], sparc) or
after it (variant 1, arm{32,64}, ppc, mips, ia64, riscv).

We can't make our pthread_internal_t exactly follow the ordinary arm64
ABI (at least) because TP[5] is used for clang's -fstack-protector on
Android. Instead, reserve extra space after the TP (up to 16 words), which
will be followed by the executable's TLS segment.

Bug: http://b/78026329
Test: boot device, bionic unit tests
Change-Id: I0f3b270b793f9872ba0effeac03f4dec364438d6