sandbox: Adjust how OS-interface files are built

sandbox: Adjust how OS-interface files are built

The current mechanism uses a completely separate build rule for each
file which must be built with system headers. This is tricky to
maintain.

Add a foreach template in the sandbox cpu Makefile which generates the
custom compile rules from a CFLAGS_USE_SYSHDRS list. This keeps the
rules data-driven without needing changes to the common
scripts/Makefile.lib, which could affect other architectures.

Move initjmp.o into the template since it uses the same pattern. Add
sdl.o to the list too, with an override for its command since it also
needs -fshort-wchar removed and -fno-lto added.

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