[libcxx] String format class marked as packed

This patch marks the class _Flags as packed because the design assumes that it
is packed and a number of tests also assume that it is packed. However on AIX
the class is not packed unless it is marked as such.

Reviewed By: hubert.reinterpretcast, #libc, Mordante, ldionne, Quuxplusone

Differential Revision: https://reviews.llvm.org/D119567