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.

libphobos/testsuite/lib/libphobos.exp

@@ -234,7 +234,7 @@ proc libphobos_init { args } {
 	    if { "$mldir" == "." } {
 	      continue
 	    }
-	    if { [llength [glob -nocomplain ${gccdir}/${mldir}/libgcc_s*.so.*]] >= 1 } {
+	    if { [llength [glob -nocomplain ${gccdir}/${mldir}/libgcc_s*.${shlib_ext}*]] >= 1 } {
 	      append ld_library_path ":${gccdir}/${mldir}"
 	    }
 	  }

libphobos/testsuite/testsuite_flags.in

@@ -36,8 +36,7 @@ case ${query} in
       ;;
     --gdcpaths)
       GDCPATHS_default="-nostdinc"
-      GDCPATHS_config="-B${BUILD_DIR}/src
-                       -I${BUILD_DIR}/libdruntime
+      GDCPATHS_config="-I${BUILD_DIR}/libdruntime
                        -I${SRC_DIR}/libdruntime"
       # Include phobos in search path if compiling in library.
       if [ "x@ENABLE_LIBDRUNTIME_ONLY_FALSE@" = "x" ]; then
@@ -48,8 +47,7 @@ case ${query} in
     --gdcldflags)
       GDCLDFLAGS="-B${BUILD_DIR}/src
 		  -B${BUILD_DIR}/libdruntime/gcc
-                  -B${BUILD_DIR}/src/.libs
-                  -L${BUILD_DIR}/src/.libs"
+                  -B${BUILD_DIR}/src/.libs"
       echo ${GDCLDFLAGS}
       ;;
     *)