Install strings unconditionally

Previously it was enabled by WITH_/WITHOUT_TOOLCHAIN, but it is commonly
expected to be available and may have non-toolchain consumers. As it
is now taken from the BSD-licensed ELF Tool Chain project, just install
it unconditionally.

PR: 213665, 223725
Reviewed by: bdrewery
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D8398