Remove incorrect platform check safe_conversions

There was a typo in arch64, the intention is aarch64,
and the implementation uses ssat/usat, which is arm (32-bit)
specific, and would not work for aarch64. So remove the
check (and support) for aarch64.

Change-Id: I9985c0c370781a8dcaccf55e2b6f182421848c97
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2440971
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815888}

base/numerics/safe_conversions.h

@@ -13,7 +13,7 @@
 
 #include "base/numerics/safe_conversions_impl.h"
 
-#if !defined(__native_client__) && (defined(__ARMEL__) || defined(__arch64__))
+#if defined(__ARMEL__) && !defined(__native_client__)
 #include "base/numerics/safe_conversions_arm_impl.h"
 #define BASE_HAS_OPTIMIZED_SAFE_CONVERSIONS (1)
 #else