Add cpu-aarch64-fuchsia.c

Fuchsia/Zircon recently added support for exposing arm64 CPU features;
this CL uses the new system call to set OPENSSL_armcap_P.

Change-Id: I045dc0b58117afe6dae315a82bf9acfd8d99be1a
Reviewed-on: https://boringssl-review.googlesource.com/25865
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

crypto/cpu-aarch64-linux.c

diff --git a/crypto/cpu-aarch64-linux.c b/crypto/cpu-aarch64-linux.c
index f9fa6c5..0184dd4 100644
--- a/crypto/cpu-aarch64-linux.c
+++ b/crypto/cpu-aarch64-linux.c
@@ -14,7 +14,8 @@
 
 #include <openssl/cpu.h>
 
-#if defined(OPENSSL_AARCH64) && !defined(OPENSSL_STATIC_ARMCAP)
+#if defined(OPENSSL_AARCH64) && defined(OPENSSL_LINUX) && \
+    !defined(OPENSSL_STATIC_ARMCAP)
 
 #include <sys/auxv.h>