Prefer AES-GCM when hardware support is available.

BUG=396787

Change-Id: I72ddb0ec3c71dbc70054403163930cbbde4b6009
Reviewed-on: https://boringssl-review.googlesource.com/1581
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/include/openssl/cpu.h b/include/openssl/cpu.h
index bec157f..3cc1e5e 100644
--- a/include/openssl/cpu.h
+++ b/include/openssl/cpu.h
@@ -79,7 +79,10 @@
  *   Index 1:
  *     ECX for CPUID where EAX = 1
  *   Index 2:
- *     EBX for CPUID where EAX = 7 */
+ *     EBX for CPUID where EAX = 7
+ *
+ * Note: the CPUID bits are pre-adjusted for the OSXSAVE bit and the YMM and XMM
+ * bits in XCR0, so it is not necessary to check those. */
 extern uint32_t OPENSSL_ia32cap_P[4];
 #endif