Add stdlib functions mbstowcs() and wcstombs().

As with the other wchar functions in Bionic, these are really
minimally functional stubs.

Change-Id: I805bc5642e7bc22d9d730cfc18f9fb4a5f164416
diff --git a/libc/stdlib/wchar.c b/libc/stdlib/wchar.c
index d805333..7722b34 100644
--- a/libc/stdlib/wchar.c
+++ b/libc/stdlib/wchar.c
@@ -227,6 +227,11 @@
     return len;
 }
 
+size_t mbstowcs(wchar_t *dst, const char *src, size_t len)
+{
+    return mbsrtowcs(dst, &src, len, NULL);
+}
+
 wint_t  putwc(wchar_t wc, FILE *stream)
 {
     return fputc((char)wc, stream);
@@ -339,6 +344,11 @@
     return len;
 }
 
+size_t wcstombs(char *dst, const wchar_t *src, size_t len)
+{
+    return wcsrtombs(dst, &src, len, NULL);
+}
+
 size_t wcsspn(const wchar_t *ws1, const wchar_t *ws2)
 {
     return strspn( (const char*)ws1, (const char*)ws2 );