[libc][CMake] Place archives in build/lib/<target-triple>

This patch moves the location of libllvmlibc.a within the build tree to
within ./lib/<target triple>. This more closely matches the behavior of
other runtime builds and allows for clang in the same build tree to
automatically be able to link against llvmlibc since this path is by
default included by the driver.

Also removes the LIBC_BINARY_DIR CMake flag since it isn't used anywhere
in the tree (based on a quick grep).

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D151624