[CMake] Support feeding DYLD_LIBRARY_PATH into archiver calls

OS X 10.11 has a feature named System Integrity Protection. The goal of the feature is to make system binaries immutable (even as root). One part of this is that protected binaries do not receive DYLD_* environment variables because the kernel scrubs them before process launch.

This causes problems for LTO bootstrap builds on Darwin that try to use the just-built libLTO with the host ar, ranlib, or libtool.

This patch addresses two problems.

(1) The tools themselves aren't protected binaries but the shim tools installed at / are, so we need to call xcrun -find to find libtool instead of using the one CMake finds.

(2) Some build tools (ninja and make) use /bin/sh to invoke their subprocesses. Since /bin/sh is a system binary, the kernel scrubs the DYLD envars from their environment. To work around this we need to set the environment variables as part of the archiver commands, so the envars are set by the shell process instead of on the shell process.

llvm-svn: 276710