add 64-bit bionic implementation for denver arch

Add 64-bit bionic implementation for denver. memcpy/memset are
denver-specific optimized. Use generic version of other routines.

Change-Id: I44a830e07b82b2986001d73d1540b4080aaa839b
diff --git a/libc/arch-arm64/arm64.mk b/libc/arch-arm64/arm64.mk
index 2c90cd6..8920f5f 100644
--- a/libc/arch-arm64/arm64.mk
+++ b/libc/arch-arm64/arm64.mk
@@ -58,7 +58,7 @@
 endif
 cpu_variant_mk := $(LOCAL_PATH)/arch-arm64/$(TARGET_CPU_VARIANT)/$(TARGET_CPU_VARIANT).mk
 ifeq ($(wildcard $(cpu_variant_mk)),)
-$(error "TARGET_CPU_VARIANT not set or set to an unknown value. Possible values are generic, generic-neon. Use generic for devices that do not have a CPU similar to any of the supported cpu variants.")
+$(error "TARGET_CPU_VARIANT not set or set to an unknown value. Possible values are generic, generic-neon, denver64. Use generic for devices that do not have a CPU similar to any of the supported cpu variants.")
 endif
 include $(cpu_variant_mk)
 libc_common_additional_dependencies += $(cpu_variank_mk)