Set the soname when creating a shared lib

It's still possible to override it, just use -optl-Wl,-soname, eg:
ghc -shared -dynamic foo.o -o libfoo.so -optl-Wl,-soname,libbar.so

compiler/main/DriverPipeline.hs

@@ -1661,6 +1661,7 @@ linkDynLib dflags o_files dep_packages = do
 	    md_c_flags
 	 ++ o_files
 	 ++ [ "-shared", "-Wl,-Bsymbolic" ] -- we need symbolic linking to resolve non-PIC intra-package-relocations
+         ++ [ "-Wl,-soname," ++ takeFileName output_fn ] -- set the library soname
 	 ++ extra_ld_inputs
 	 ++ lib_path_opts
 	 ++ extra_ld_opts