[PATCH] [libcxx] Update libc++ docs to include instructions for LIT.

[Ed Maste]

@danalbert: For background, we switched to Clang + libc++ in FreeBSD 10.0 (released Jan 2014) on i386/amd64/arm.  GCC and libstdc++ are not installed by default.  Work is ongoing for mips, powerpc, and sparc.

http://reviews.llvm.org/D4766