[libc++] Build the dylib with -faligned-allocation

This avoids issues when building the dylib for deployment targets that
don't support aligned allocation, where Clang normally triggers an
error to warn users their code would break at runtime when back-deployed.
Since we're building the dylib itself, which contains the aligned
allocation functions, we don't want to trigger that error.

Differential Revision: https://reviews.llvm.org/D84418