testsuite, gfortran: Update link flags [PR112862].

The regressions here are caused by two issues:
1. In some cases there is no generated runpath for libatomic
2. In other cases there are duplicate paths.

This patch simplifies the addition of the options in the main
gfortran exp and removes the duplicates elewhere.

We need to add options to locate libgfortran and the dependent libs
libquadmath (supporting REAL*16) and libatomic (supporting operations
used by coarrays).  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.

	PR target/112862

gcc/testsuite/ChangeLog:

	* gfortran.dg/coarray/caf.exp: Remove duplicate additions of
	libatomic handling.
	* gfortran.dg/dg.exp: Likewise.
	* lib/gfortran.exp: Decide on whether to present -B or -L to
	reference the paths to uninstalled libgfortran, libqadmath and
	libatomic and use that to generate the link flags.