build: Enable x86 assembler on Hurd.

Taken from downstream:
http://anonscm.debian.org/gitweb/?p=pkg-xorg/lib/mesa.git;a=blob;f=debian/patches/10-hurd-configure-tweaks.diff;h=984e17df1b8afdf8e4b36bee96aa5ab6a5691021;hb=refs/heads/ubuntu%2B1

Thanks to Pino Toscano.

v2: Don't bother with x86_64. AFAICT GNU/Hurd doesn't support it so far.

NOTE: This is a candidate for stable branches.

Acked-by: Kenneth Graunke <kenneth@whitecape.org> (v1)
Acked-by: Matt Turner <mattst88@gmail.com>
diff --git a/configure.ac b/configure.ac
index 4b5b045..81d4a3f 100644
--- a/configure.ac
+++ b/configure.ac
@@ -452,6 +452,9 @@
         linux* | *freebsd* | dragonfly* | *netbsd*)
             test "x$enable_64bit" = xyes && asm_arch=x86_64 || asm_arch=x86
             ;;
+        gnu*)
+            asm_arch=x86
+            ;;
         esac
         ;;
     x86_64)