Allow make-link-multidir to make subdirectories

The RISC-V Linux ABI doesn't define any libraries that go directly in
lib, instead they go into lib32/ilp32 or lib64/lp64.  This casuse
make-link-multidir to fail when attempting to make library directories
when building a static libc on multilib RISC-V systems.

This patch uses scripts/mkinstalldirs to make the base directory of the
target symlink of make-link-multidir.

2018-01-06  Palmer Dabbelt  <palmer@sifive.com>

       * Makerules (make-link-multidir): Make directories before linking into
       them.