Add optimized stpcpy.

Change-Id: Ifed38f92a54fef4488fd0ed26a1569059a054574
diff --git a/libc/arch-arm64/generic-neon/generic-neon.mk b/libc/arch-arm64/generic-neon/generic-neon.mk
index b464d96..f3bde5c 100644
--- a/libc/arch-arm64/generic-neon/generic-neon.mk
+++ b/libc/arch-arm64/generic-neon/generic-neon.mk
@@ -2,6 +2,7 @@
     arch-arm64/generic/bionic/memcmp.S \
     arch-arm64/generic/bionic/memmove.S \
     arch-arm64/generic/bionic/memset.S \
+    arch-arm64/generic/bionic/stpcpy.S \
     arch-arm64/generic/bionic/strchr.S \
     arch-arm64/generic/bionic/strcmp.S \
     arch-arm64/generic/bionic/strcpy.S \