fix mismatched signatures for strtod_l family

fix mismatched signatures for strtod_l family

strtod_l, strtof_l, and strtold_l originally existed only as
glibc-ABI-compat symbols. as noted in the commit which added them,
17a60f9d327c6f8b5707a06f9497d846e75c01f2, making them aliases for the
non-_l functions was a hack and not appropriate if they ever became
public API.

unfortunately, commit 35eb1a1a9b97577e113240cd65bf9fc44b8df030 did
make them public without undoing the hack. fix that now by moving the
the _l functions to their own file as wrappers that just throw away
the locale_t argument.