Handle host_cpu of x86_64 like other x86s.

diff --git a/configure.ac b/configure.ac
index a1deb93..e6081e0 100644
--- a/configure.ac
+++ b/configure.ac
@@ -49,6 +49,7 @@
       test -n "$CC" || CC="gcc -m32"
       test -n "$LD" || LD="ld -m elf_i386"
       test -n "$AS" || AS="as --32"
+      host_cpu="i386"
     fi
     ;;
 esac