Add sysroot/lib to library search path of baremetal toolchain.

Baremetal toolchain is not adding sysroot/lib to the library
search path. This is forcing the user to do it manually. This commit
fixes this shortcoming by adding the sysroot/lib to library search path
if sysroot is not empty.

Reviewed By: jroelofs

Differential Revision: https://reviews.llvm.org/D91559