man: Conditionally install ar.5 in toolchain package

Previously if WITHOUT_TOOLCHAIN was set we'd create a FreeBSD-clang
package that contained only ar.5. As ar.5 describes the ar format
and doesn't come from Clang/LLVM move it to the FreeBSD-toolchain
package and make it conditional on MK_TOOLCHAIN.

Reviewed by: ivy
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52886

(cherry picked from commit a8d774d9708c100a52f231065d9d5f1b4c3aceb7)