sim: Fix use of out-of-tree assembler and linker when testing

sim: Fix use of out-of-tree assembler and linker when testing

With commit 7a259895bb2d "sim: testsuite: expand arch specific
toolchain settings", trying to use out-of-tree ld and as at test-time
broke for the "primary target", like when testing a release-tarball.

Subsequent to that commit, all assembler tests without in-tree-built
tools FAIL, getting errors when trying to call
$(abs_builddir)/../gas/as-new.  But, that isn't the actual culprint;
it's actually it's its immediate predecessor, commit 8996c21067373
"sim: testsuite: setup per-port toolchain settings for multitarget
build", which hardcodes in-tree-paths to those tools instead of
considering e.g. $(<X>_FOR_TARGET), the preferred overridable variable
for single-target builds, as set up by the toplevel Makefile.

This commit calls GCC_TARGET_TOOL (a deceptive name; gcc-specific
features aren't used) from toplev/config/acx.m4, somewhat like calls
in toplev/configure.ac but without the NCN_STRICT_CHECK_TARGET_TOOLS
step, for each X to find a value for $(<X>_FOR_TARGET).  N.B.: in-tree
tools still override any ${target}-${tool} found in $PATH, i.e. only
previously broken builds are affected.

The variables $(<X>_FOR_TARGET) are usually overridden by the toplevel
Makefile to the same value or better, but has to be set here too, as
automake "wants" Makefiles to be self-contained (you get an error
pointing out that the variable may be empty).  If it hadn't been for
that, SIM_AC_CHECK_TOOLCHAIN_FOR_PRIMARY_TARGET would not be needed.
This detail should only (positively) affect users invoking "make
check" in sim/ instead of "make check-sim" (or "make check") at the
toplevel.  Now the output from "configure" matches the target tools
actually used by sim at test-time, for the "primary target".

Using $(CC) for "example-" targets CC_FOR_TARGET is not changed, as
that appears to be a deliberate special-case.

Note that all tools still have to be installed and present in
$PATH at configure-time to be properly used at test-time.

sim:
	* m4/sim_ac_toolchain.m4 (SIM_AC_CHECK_TOOLCHAIN_FOR_PRIMARY_TARGET):
	New defun.
	(SIM_TOOLCHAIN_VARS): Call it using AC_REQUIRE, and use variables
	AS_FOR_TARGET, LD_FOR_TARGET and CC_FOR_TARGET instead of hard-coded
	values.
	* Makefile.in, configure: Regenerate.