Make AARCH64 harder, better, faster, stronger

- Perform some housekeeping on scalar math function code
- Import ARM's Optimized Routines for SIMD string processing
- Upgrade to latest Chromium zlib and enable more SIMD optimizations

libc/tinymath/fdim.c

@@ -22,6 +22,6 @@
  * Returns positive difference.
  */
 double fdim(double x, double y) {
-  if (isnan(x) || isnan(y)) return NAN;
+  if (isunordered(x, y)) return NAN;
   return x > y ? x - y : 0;
 }