locale: Use compat_symbol_reference in _nl_postload_ctype

These symbol usages are not definitions, so compat_symbol_reference is
more appropriate than compat_symbol.  compat_symbol_reference is also
safe to emit multiple times (in case the inline assembly is
duplicated; this is possible because it is nested in a function).
compat_symbol does not necessarily have this property because it is
intended to provide a symbol definition.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>