sim: unify target->subdir handling for default tests

sim: unify target->subdir handling for default tests

The testsuite subdir has a note about unifying the target->subdir logic,
so do just that.  The end goal here is to have `make check` work out of
the box without having to delve into dejagnu internals.

The target-specific logic is split out of the top level configure.ac file
and into a dedicated configure.tgt similar to other subprojects (gdb and
ld and etc...) with the difference that this file has to be included at
the m4 level instead of the shell level.  This is necessary only because
autoconf requires AC_CONFIG_SUBDIRS be given a string literal and not a
variable value.

Then the toplevel and the testsuite configure files pull this in, the sim
subdir gets expanded into testsuite/site.exp, and the default sim run code
uses this info to set the sim path to the local compiled run file if it
hasn't already been specified.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>