stdlib: Use __get_numeric_locale instead of __localeconv_l for decimal_point

stdlib: Use __get_numeric_locale instead of __localeconv_l for decimal_point

The string/float conversion functions need to get the locale decimal
point. Instead of calling __localeconv_l (which copies locale data
into lconv form from __get_numeric_locale), use __get_numeric_locale
directly.

Signed-off-by: Keith Packard <keithp@keithp.com>