ldbl-opt: Reuse test cases from misc/ that check long double

This patch adds test cases for the compatibility versions of the
functions: err, errx, verr, verrx, warn, warnx, vwarn, vwarnx (from
err.h), error, and error_at_line (from error.h), when long double has
the same format as double (-mlong-double-64).

Tested for powerpc, powerpc64 and powerpc64le.