[libc++] Add a job running GCC with C++11

This configuration is interesting because GCC has a different level of
strictness for some C++ rules. In particular, it implements the older
standards more stringently than Clang, which can help find places where
we are non-conforming (especially in the test suite).

Differential Revision: https://reviews.llvm.org/D105936

GitOrigin-RevId: 851a335b1e64d0d32e15a8d2ace48c9b0967cd5a
4 files changed