libc: add mempcpy(3) and wmempcpy(3)

Reviewed by:	markj
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D31180

include/string.h

@@ -66,6 +66,9 @@ void	*memcpy(void * __restrict, const void * __restrict, size_t);
 void	*memmem(const void *, size_t, const void *, size_t) __pure;
 #endif
 void	*memmove(void *, const void *, size_t);
+#if __BSD_VISIBLE
+void	*mempcpy(void * __restrict, const void * __restrict, size_t);
+#endif
 void	*memset(void *, int, size_t);
 #if __POSIX_VISIBLE >= 200809
 char	*stpcpy(char * __restrict, const char * __restrict);

include/wchar.h

@@ -172,6 +172,9 @@ wchar_t	*wmemchr(const wchar_t *, wchar_t, size_t) __pure;
 int	wmemcmp(const wchar_t *, const wchar_t *, size_t) __pure;
 wchar_t	*wmemcpy(wchar_t * __restrict, const wchar_t * __restrict, size_t);
 wchar_t	*wmemmove(wchar_t *, const wchar_t *, size_t);
+#if __BSD_VISIBLE
+wchar_t	*wmempcpy(wchar_t * __restrict, const wchar_t * __restrict, size_t);
+#endif
 wchar_t	*wmemset(wchar_t *, wchar_t, size_t);
 int	wprintf(const wchar_t * __restrict, ...);
 int	wscanf(const wchar_t * __restrict, ...);