cross-build: Provide mempcpy when building on macOS

We could patch the tzcode config to not use it, but it's simple to
provide an implementation of it and avoid spreading cross-build
bootstrapping special cases.

Fixes:		ff2c98b30b ("tzcode: Update to 2026a")
MFC after:	1 week

(cherry picked from commit 47402c9422)

tools/build/Makefile

@@ -183,6 +183,13 @@ SRCS+=	progname.c
 # Stub implementations of fflagstostr/strtofflags
 SRCS+=	fflags.c
 .endif
+
+.if ${.MAKE.OS} == "Darwin"
+# macOS currently lacks mempcpy
+.PATH: ${LIBC_SRCTOP}/string
+SRCS+=	mempcpy.c
+.endif # ${MAKE.OS} == "Darwin"
+
 # Provide the same arc4random implementation on Linux/macOS
 CFLAGS.arc4random.c+=	-I${SRCTOP}/sys/crypto/chacha20 -D__isthreaded=1
 SRCS+=	arc4random.c arc4random_uniform.c

tools/build/cross-build/include/mac/string.h

@@ -37,6 +37,8 @@
 
 #include_next <string.h>
 
+void	*(mempcpy)(void * __restrict, const void * __restrict, size_t);
+
 /*
  * strchrnul is provided by macOS 15.4 and later.  However, there is
  * no good way to detect the current host version at compile time, so