libc/glibc: correctly handle dual-bitness archs

If the selected ARCH is dual-bitness (eg. supports 32- and 64-bit),
then we need to know the correct place where to fetch some headers.
Currently, this applies only to x86 variants: i386 and x86_64.

 #!/bin/sh

 # Needed for systems without gettext

 $* -xc -o /dev/null - > /dev/null 2>&1 << EOF

 #include <libintl.h>

 int main()

 {

 	gettext("");

 	return 0;

 }

 EOF

 if [ ! "$?" -eq "0"  ]; then

 	echo -DKBUILD_NO_NLS;

 fi