Attach the cortex strings library to the build. Only a subset of functions

have been added as some don't seem to be improvements over the libc C
implementation.

Obtained from:	ABT Systems Ltd
Sponsored by:	The FreeBSD Foundation

lib/libc/aarch64/string/Makefile.inc

@@ -0,0 +1,19 @@
+# $FreeBSD$
+#
+# String handling from the Cortex Strings library
+# https://git.linaro.org/toolchain/cortex-strings.git
+#
+
+.PATH: ${LIBC_SRCTOP}/../../contrib/cortex-strings/src/aarch64
+
+MDSRCS+=memchr.S \
+	memcmp.S \
+	memcpy.S \
+	memmove.S \
+	memset.S \
+	strchr.S \
+	strcmp.S \
+	strcpy.S \
+	strlen.S \
+	strncmp.S \
+	strnlen.S