usr.bin/Makefile: always build ELF Tool Chain strings(1)

strings(1) is not conditional on WITH_/WITHOUT_TOOLCHAIN, as it is a
small utility that is also useful outside of the toolchain context.
As of commit 1cae7121c667 we switched to WITH_LLVM_BINUTILS by default.

After this change building world with default options but installing
WITHOUT_TOOLCHAIN failed, because we would build LLVM's strings but
attempt to install ELF Tool Chain's version, which did not exist.
Address this by always including ELF Tool Chain strings in non-install
make targets, so that it will be available if options are changed at
install time.

PR: 285556
Reported by: Michael Butler
Reviewed by: brooks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49425

(cherry picked from commit fdc4db57224ce19b867c60fce4c410068be40c27)