sim: arm: move build logic to source files

This simplifies the build logic a bit by just having source file
inputs.  It also simplifies code that assumes there's a source
file for each object.

sim/arm/armemu32.c

@@ -0,0 +1,18 @@
+/* Build armemu.c with ARM32 support.
+   Copyright (C) 1995-2021 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program; if not, see <https://www.gnu.org/licenses/>.  */
+
+#define MODE32
+#include "armemu.c"