[scudo] Bump the Android API level requirement to 21 for getauxval

[cryptoad]

Summary

getauxval was introduced in 18 & 21 depending on the architecture. Bump the
requirement to 21.

It also turns out that the NDK is finicky: NDK r13b doesn't include sys/auxv.h
when creating a standalone toolchain at API level 19 for ARM. So 18 didn't work
well with older NDKs.

lib/scudo/scudo_utils.cpp

 
 #include "scudo_utils.h"
 
 #include <stdarg.h>
 #if defined(__x86_64__) || defined(__i386__)
 # include <cpuid.h>
 #endif
 #if defined(__arm__) || defined(__aarch64__)
-# if SANITIZER_ANDROID && __ANDROID_API__ < 18
-// getauxval() was introduced with API level 18 on Android. Emulate it using
-// /proc/self/auxv for lower API levels.
+# if SANITIZER_ANDROID && __ANDROID_API__ < 21
+// getauxval() was introduced with API level 18 for ARM and 21 for AArch64.
+// Emulate it using /proc/self/auxv for lower API levels.
 #  include "sanitizer_common/sanitizer_posix.h"
 
 #  include <fcntl.h>
 
 #  define AT_HWCAP 16
 
 namespace __sanitizer {