libc-locale: fix ASCII compatible warning cause build failure.

libc-locale: fix ASCII compatible warning cause build failure.

Some locales are not listed in glibc locales support list, but can be generated,
here using ja_JP.SHIFT_JIS as an example. We can add following line into local.conf
to enable and generate it:

GLIBC_GENERATE_LOCALES += "en_GB.UTF-8 en_US.UTF-8 ja_JP.UTF-8 ja_JP.SHIFT_JIS"
IMAGE_LINGUAS += "ja-jp en-us ja-jp.shift-jis"

The localedef tool would report a warning and exit with 1, that cause build failure,
error message as below:

[warning] character map `SHIFT_JIS' is not ASCII compatible, locale not ISO C compliant [--no-warnings=ascii]

So add a --no-warnings=ascii in libc-package.bbclass to fix build failure if someone needs those locale
in yocto.

Signed-off-by: Xiangyu Chen <xiangyu.chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>