libc: Fix the alignment of the TCB to match rtld for several architectures.

• Use 16 byte alignment rather than 8 for aarch64, powerpc64, and RISC-V.

• Use 8 byte alignment rather than 4 for 32-bit arm, mips, and powerpc.

I suspect that mips64 should be using 16 byte alignment, but both libc
and rtld currently use 8 byte alignment.

Reviewed by: kib, jrtc27
Sponsored by: The University of Cambridge, Google Inc.
Differential Revision: https://reviews.freebsd.org/D33350