linux-user: Create target/elfload.c files

Prepare to split the main linux-user/elfload.c.
Create empty files for each target, and add the common build rule.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

linux-user/microblaze/elfload.c

@@ -0,0 +1 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */