sna: Rewrite __fls without dependence upon x86 assembly

The asm() prevents SNA from compiling on ia64.

Fixes https://bugs.gentoo.org/show_bug.cgi?id=448570

src/sna/kgem.c

@@ -516,10 +516,19 @@ static void gem_close(int fd, uint32_t handle)
 
 constant inline static unsigned long __fls(unsigned long word)
 {
+#if defined(__GNUC__) && (defined(__x86__) || defined(__x86_64__))
 	asm("bsr %1,%0"
 	    : "=r" (word)
 	    : "rm" (word));
 	return word;
+#else
+	unsigned int v = 1;
+
+	while (word >>= 1)
+		v++;
+
+	return v;
+#endif
 }
 
 constant inline static int cache_bucket(int num_pages)