subst.mk: Fix mk/scrips/subst-identity.awk for FreeBSD

At least on FreeBSD 13.0, awk '/^[\t -~]/' does not match alphabetical
characters with some utf-8 locales (e.g. neither en_US.UTF-8 nor
fr_FR.UTF-8 works but C.UTF-8 does).

(triaxx)