bionic: add support for non-NEON memcpy() on NEON SoCs

Some SoCs that support NEON nevertheless perform better with a non-NEON than a
NEON memcpy().  This patch adds build variable ARCH_ARM_USE_NON_NEON_MEMCPY,
which can be set in BoardConfig.mk.  When ARCH_ARM_USE_NON_NEON_MEMCPY is
defined, we compile in the non-NEON optimized memcpy() even if the SoC supports
NEON.

Change-Id: Ia0e5bee6bad5880ffc5ff8f34a1382d567546cf9
diff --git a/libc/arch-arm/bionic/memcpy.S b/libc/arch-arm/bionic/memcpy.S
index 438fa00..8c0b4d7 100644
--- a/libc/arch-arm/bionic/memcpy.S
+++ b/libc/arch-arm/bionic/memcpy.S
@@ -29,7 +29,7 @@
 #include <machine/cpu-features.h>
 #include <machine/asm.h>
 
-#if defined(__ARM_NEON__)
+#if defined(__ARM_NEON__) && !defined(ARCH_ARM_USE_NON_NEON_MEMCPY)
 
         .text
         .fpu    neon