[libc++] Add missing __format__ attributes

-Wformat-nonliteral was turned on in https://reviews.llvm.org/D112927,
however we forgot to apply some __format__ attributes in Linux specific
code paths, which led to warnings when building on Linux. This patch
addresses that oversight.

Differential Revision: https://reviews.llvm.org/D113876

GitOrigin-RevId: 7dc9a03cfd789e6a08ca96666e9fbb81431eb34f
3 files changed