testsuite, GDC: Update link flags [PR112861].

The regressions here are because we do not generate a runpath for
the uninstalled libstdc++.  This patch updates the link flags handling
to simplify it.

We need to add options to locate both libgphobos and libstdc++
Usually '-L' options are added to point to the relevant directories for
the uninstalled libraries.

In cases where libraries are available as both shared and convenience
some additional checks are made.

For some targets -static-xxxx options are handled by specs substitution
and need a '-B' option rather than '-L'.  For Darwin, when embedded
runpaths are in use (the default for all versions after macOS 10.11),
'-B' is also needed to provide the runpath.

When '-B' is used, this results in a '-L' for each path that exists (so
that appending a '-L' as well is a needless duplicate).  There are also
cases where tools warn for duplicates, leading to spurious fails.
Therefore the objective is to add a single -B/-L option for each needed
path.

	PR target/112861

gcc/testsuite/ChangeLog:

	* lib/gdc.exp: Decide on whether to present -B or -L to reference
	the paths to uninstalled libphobos and libstdc++ and use that to
	generate the link flags.

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