sim: better handle builds of primary targets lacking sims

When building with a primary target that doesn't feature a simulator,
one would expect for nothing to be done in sim/.  However, a
$(top_builddir)/sim/testsuite directory is created, with a Makefile
containing a rule like:

check-DEJAGNU: site.exp
	echo "Dejagnu-checking in `pwd` directory ..."
	rootme=`pwd`; export rootme; echo rootme = $$rootme; \
	srcdir=`cd ${srcdir}; pwd`; export srcdir ; echo srcdir = $$srcdir; \
	EXPECT=${EXPECT} ; export EXPECT ; echo EXPECT = $$EXPECT; \
	if [ -f $$rootme/../../expect/expect ]; then \
	  TCL_LIBRARY=`cd $$srcdir/../../tcl/library && pwd`; \
	  export TCL_LIBRARY; \
	fi; \
        echo TCL_LIBRARY = $$TCL_LIBRARY; \
	runtest=$(RUNTEST); echo runtest = $$runtest; \
	if $(SHELL) -c "$$runtest --version" > /dev/null 2>&1; then \
	  $$runtest $(RUNTESTFLAGS); \
	else echo "WARNING: could not find \`runtest'" 1>&2; :;\
	fi

Consequently, when `make check' recurses into sim/testsuite, the above
rule is executed.  Until now, the desired effect (of doing nothing)
was achieved because `runtest --version' fails due to a malformed
site.exp being generated in objdir: it is malformed because the
primary target doesn't configure a $sim_arch. i.e. this was doing the
right thing just by chance.

However, the git version of dejagnu seems to have changed in a way
runtest doesn't try to load site.exp when it gets --version.  The net
effect is that the rule above tries to actually run the tests, failing
miserably.

This little patch makes sim/configure to not recurse into
sim/testsuite if the primary target didn't configure a simulator.

Tested with:
- A simulator target (bpf-unkonwn-none).
- A simulator-less target (x86_64-linux-gnu).
- A simulator-less target and --build-targets=all.

sim/ChangeLog:

2020-09-03  Jose E. Marchesi  <jose.marchesi@oracle.com>

	* configure.ac: Do not configure sim/testsuite nor sim/igen if the
	primary target doesn't have a simulator.
	* configure: Regenerate.

sim/configure.ac

@@ -43,9 +43,11 @@ m4_define([SIM_ARCH], [
 ])
 if test "${enable_sim}" != no; then
    sinclude(configure.tgt)
-   AC_CONFIG_SUBDIRS(testsuite)
-   if test "$sim_igen" = yes; then
-      AC_CONFIG_SUBDIRS(igen)
+   if test x"${sim_arch}" != x; then
+      AC_CONFIG_SUBDIRS(testsuite)
+      if test "$sim_igen" = yes; then
+         AC_CONFIG_SUBDIRS(igen)
+      fi
    fi
 fi