ldbl-opt: Add error and error_at_line (bug 23984)

On platforms where long double may have the same format as double
(-mlong-double-64), error and error_at_line do not take that into
account and might produce wrong output if a long double conversion is
requested by the format string ('%Lf').  This patch adds compatibility
functions for this situation and redirects calls via header magic.

Tested for powerpc, powerpc64 and powerpc64le.