contrib: Use config.sub in test_installed

contrib: Use config.sub in test_installed

Correctly running GCC testsuites with an installed compiler requires
both the canonical and noncanonical versions of the target triplet:
the canonical one for where the testsuite matches on target triplets,
and the noncanonical one for various "transform" calls used to find
binutils programs.  Make test_installed use config.sub to determine
the canonical target from any value passed with --target= (and thus
make logic to locate the toplevel source directory from its own
location unconditional, as it's now used to locate config.sub).

	* test_installed: Use config.sub to determine canonical target.