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arm: Improve use of getauxval in quirky environments

Manually define the necessary HWCAP if sys/auxv.h didn't define
it. (It isn't defined in the Android NDK, one has to include
asm/hwcap.h to get definitions of them. Additionally, the aarch64
cross libc headers in Ubuntu 16.04 lacked HWCAP definitions from
sys/auxv.h, also present in asm/hwcap.h. Ubuntu 16.04's armhf
cross libc headers did however define HWCAP values in sys/auxv.h.)

Don't use any detection at all for NEON on aarch64, since it is a
mandatory feature there. On WSL on Windows 10 1803 arm64,
getauxval(AT_HWCAP) returns 0.

--- a/src/arm/cpu.c

+++ b/src/arm/cpu.c

@@ -29,14 +29,13 @@

 #include "src/arm/cpu.h"

-#if defined(HAVE_GETAUXVAL)

+#if defined(HAVE_GETAUXVAL) && ARCH_ARM

 #include <sys/auxv.h>

-#if ARCH_AARCH64

-#define NEON_HWCAP HWCAP_ASIMD

-#elif ARCH_ARM

-#define NEON_HWCAP HWCAP_ARM_NEON

+#ifndef HWCAP_ARM_NEON

+#define HWCAP_ARM_NEON (1 << 12)

 #endif

+#define NEON_HWCAP HWCAP_ARM_NEON

 #elif defined(__ANDROID__)

 #include <stdio.h>

@@ -71,7 +70,9 @@

 unsigned dav1d_get_cpu_flags_arm(void) {

     unsigned flags = 0;

-#if defined(HAVE_GETAUXVAL)

+#if ARCH_AARCH64

+    flags |= DAV1D_ARM_CPU_FLAG_NEON;

+#elif defined(HAVE_GETAUXVAL) && ARCH_ARM

     unsigned long hw_cap = getauxval(AT_HWCAP);

     flags |= (hw_cap & NEON_HWCAP) ? DAV1D_ARM_CPU_FLAG_NEON : 0;

 #elif defined(__ANDROID__)