We assume that AArch64 systems are modern enough to have getauxval.

Change-Id: I4cc74f04ca5ed50b8ca1cfd00afeaaa01c6caca0
Reviewed-on: https://boringssl-review.googlesource.com/3280
Reviewed-by: Adam Langley <agl@google.com>

Android didn't have getauxval until Jelly Bean (4.1). This means that
BoringSSL running on older Androids won't be able to detect NEON
support. (This is moot for Chromium because Chromium calls
android_getCpuFeatures and sets the NEON flag itself, but other users of
BoringSSL on Android probably won't do that.)

This change mirrors a little of what upstream does and tries running a
NEON instruction with a handler for SIGILL installed.

Change-Id: I853b85c37ffb049b240582d71fcf07adedc37a30
Reviewed-on: https://boringssl-review.googlesource.com/3190
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>