testsuite, libphobos: Update link flags [PR112864].

The regressions here are primarily from duplicated '-B' additions.

We remove the duplicate on the link line.
We also make sure that platforms with extensions other than .so for
shared libs will have the correct paths.

	PR target/112864

libphobos/ChangeLog:

	* testsuite/lib/libphobos.exp: Use ${shlib_ext} instead of
	hard-wiring '.so'.
	* testsuite/testsuite_flags.in: Remove duplicate -B option
	for spec file path.