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.

 reporter_name="Ioannis E. VENETIS"

 reporter_url="http://sourceware.org/ml/crossgcc/2009-11/msg00109.html"

 reporter_comment=""