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.

 --- binutils/ld/ldmain.c

 +++ binutils/ld/ldmain.c

 @@ -293,2 +293,3 @@ main (int argc, char **argv)

    link_info.combreloc = TRUE;

 +  link_info.relro = TRUE;

    link_info.strip_discarded = TRUE;