Fix assembler warnings.

There are a few instructions deprecated on armv8 that result in lots
of warnings. Add an arch directive so that these warnings go away.

This doesn't cause any problems because the instructions still
execute properly.

Bug: 38319728

Test: Built all of these assembler files and verified the warning are gone.
Change-Id: If063defdd16f290c01975233c8d257d1b2005e76
diff --git a/libc/arch-arm/denver/bionic/memcpy.S b/libc/arch-arm/denver/bionic/memcpy.S
index d4e0fb4..743c74b 100644
--- a/libc/arch-arm/denver/bionic/memcpy.S
+++ b/libc/arch-arm/denver/bionic/memcpy.S
@@ -61,6 +61,10 @@
         .syntax unified
         .fpu    neon
 
+        // To avoid warning about deprecated instructions, add an explicit
+        // arch. The code generated is exactly the same.
+        .arch armv7-a
+
 ENTRY(__memcpy_chk)
         cmp     r2, r3
         bls     memcpy