sandbox: Unprotect DATA regions in bus tests

On my Ubuntu 18.04.1 machine two driver-model bus tests have started
failing recently. The problem appears to be that the DATA region of the
executable is protected. This does not seem correct, but perhaps there
is a reason.

To work around it, unprotect the regions in these tests before accessing
them.

Signed-off-by: Simon Glass <sjg@chromium.org>

arch/sandbox/cpu/os.c

@@ -636,3 +636,14 @@ void os_abort(void)
 {
 	abort();
 }
+
+int os_mprotect_allow(void *start, size_t len)
+{
+	int page_size = getpagesize();
+
+	/* Move start to the start of a page, len to the end */
+	start = (void *)(((ulong)start) & ~(page_size - 1));
+	len = (len + page_size * 2) & ~(page_size - 1);
+
+	return mprotect(start, len, PROT_READ | PROT_WRITE);
+}