[AARCH64] correct alignment of TLS_TCB_ALIGN (BZ #16796)

This fixes a variety of testsuite failures for me:
tststatic.out Error 1
tststatic2.out Error 1
tst-tls9-static.out Error 1
tst-audit8.out Error 127
tst-audit9.out Error 127
tst-audit1.out Error 127
and also has the added benefit of making LD_AUDIT/sotruss work on
AArch64.

Otherwise, we bail out early in _dl_try_allocate_static_tls as the
alignment requirement of the PT_TLS section in libc is 16.