libgfortran: Fix libgfortran.so versioning on Solaris with subdirs

The move of libgfortran objects to subdirectories completely broke the
creation of libgfortran.so on Solaris.  Since the gfortran.ver-sun rule
doesn't support that structure, no libtool objects are found, thus no
symbols exported from libgfortran.so, causing every link to fail.

This patch fixes this by allowing for the new structure.

Tested on i386-pc-solaris2.11 and sparc-sun-solaris2.11.

2024-05-05  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>

	libgfortran:
	* Makefile.am [LIBGFOR_USE_SYMVER_SUN] (gfortran.ver-sun): Handle
	objects in subdirs.
	* Makefile.in: Regenerate.