[CMake][libc] Don't put archive in build/lib/<target triple> by default

ea8f4b98 broke some build configurations
because it was enabled by default and some people are using a just built
libc/clang/LLVM to work on other projects where having a just built LLVM
libc in one of Clang's default include directories can make things
unusable.

Differential Revision: https://reviews.llvm.org/D152190