stdlib: conditionalize locale usage

stdlib: conditionalize locale usage

_strtod_l as well as the gethex function both fetch the decimal point
from the current LC_NUMERIC locale info.  This pulls in _C_numeric_locale
unconditionally even on targets not supporting locales at all.

Another problem is that strtod.c and gdtoa-gethex.c are ELIX 1, while
locale information in general isn't.  This leads to potential build
breakage on bare metal targets.

Fix this by setting the decimal point to "." on all targets not
defining __HAVE_LOCALE_INFO__.

While at it, const'ify the entire local decimal point info in the
affected functions.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>