dso-ordering-test.py: Put all sources in one directory [BZ #28550]

dso-ordering-test.py: Put all sources in one directory [BZ #28550]

Put all sources for DSO sorting tests in the dso-sort-tests-src directory
and compile test relocatable objects with

$(objpfx)tst-dso-ordering1-dir/tst-dso-ordering1-a.os: $(objpfx)dso-sort-tests-src/tst-dso-ordering1-a.c
	$(compile.c) $(OUTPUT_OPTION)

to avoid random $< values from $(before-compile) when compiling test
relocatable objects with

$(objpfx)%$o: $(objpfx)%.c $(before-compile); $$(compile-command.c)
compile-command.c = $(compile.c) $(OUTPUT_OPTION) $(compile-mkdep-flags)
compile.c = $(CC) $< -c $(CFLAGS) $(CPPFLAGS)

for 3 "make -j 28" parallel builds on a machine with 112 cores at the
same time.

This partially fixes BZ #28550.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>