Add mempcpy implementation

A GNU extension that returns dst + size instead of dst. It's a nice
shortcut when copying strings with a known size or back-to-back blocks
and you have to do it often.

May of course be called directly or alternately through
__builtin_mempcpy in some compiler versions.

For ASM on native targets, it is implemented as an alternate entrypoint
to memcpy which adds minimal code and overhead.

Change-Id: I4cbb3483f6df3c1007247fe0a95fd7078737462b

firmware/asm/mempcpy.c

@@ -0,0 +1,47 @@
+/*
+FUNCTION
+        <<mempcpy>>---copy memory regions and return end pointer
+
+ANSI_SYNOPSIS
+        #include <string.h>
+        void* mempcpy(void *<[out]>, const void *<[in]>, size_t <[n]>);
+
+TRAD_SYNOPSIS
+        void *mempcpy(<[out]>, <[in]>, <[n]>
+        void *<[out]>;
+        void *<[in]>;
+        size_t <[n]>;
+
+DESCRIPTION
+        This function copies <[n]> bytes from the memory region
+        pointed to by <[in]> to the memory region pointed to by
+        <[out]>.
+
+        If the regions overlap, the behavior is undefined.
+
+RETURNS
+        <<mempcpy>> returns a pointer to the byte following the
+        last byte copied to the <[out]> region.
+
+PORTABILITY
+<<mempcpy>> is a GNU extension.
+
+<<mempcpy>> requires no supporting OS subroutines.
+
+        */
+
+#include "config.h"
+#include "_ansi.h" /* for _DEFUN */
+#include <string.h>
+
+/* This may be conjoined with memcpy in <cpu>/memcpy.S to get it nearly for
+   free */
+
+_PTR
+_DEFUN (mempcpy, (dst0, src0, len0),
+        _PTR dst0 _AND
+        _CONST _PTR src0 _AND
+        size_t len0)
+{
+    return memcpy(dst0, src0, len0) + len0;
+}