testsuites/ada: Introduce init.c files

Provide an init.c for each Ada test instead of the #include <config.h>
magic which works only due to hand crafted include paths.

Update #3818.

testsuites/ada/samples/hello/Makefile.am

@@ -6,7 +6,7 @@ include $(top_srcdir)/ada.am
 ada_hello$(EXEEXT): hello.adb init.$(OBJEXT)
 	$(GNATCOMPILE) -margs -a $< -o $@
 
-init.$(OBJEXT): ../../support/init.c
+init.$(OBJEXT): init.c
 	$(COMPILE.c) $(AM_CPPFLAGS) -I$(srcdir) -c $<
 
 noinst_PROGRAMS = ada_hello

testsuites/ada/samples/hello/init.c

@@ -28,6 +28,6 @@
 
 #define ADA_TEST_NAME "ADA HELLO"
 
-#include <rtems/confdefs.h>
+#include "../../support/initimpl.h"
 
 /* end of include file */