[libc++] Ensure streams are initialized early

When statically linking libc++ on some systems, the streams are not
initialized early enough, which causes all kinds of issues. This was
reported e.g. in http://llvm.org/PR28954, but also in various open
source projects that use libc++.

Fixes http://llvm.org/PR28954.

Differential Revision: https://reviews.llvm.org/D31413