[libc++] Remove old CMake workaround
[libc++] Remove old CMake workaround

We haven't had any complaints so far, and I don't think anybody builds
libc++ from source for that old platform anymore.

git-svn-id: 756ef344af921d95d562d9e9f9389127a89a6314@355336 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx