gccrs: Add rustc test directory for testsuite adaptor

gcc/testsuite/ChangeLog:

	* rust/rustc/README.md: information about
	rustc external directory.
	* rust/rustc/rustc.exp: New test.

Signed-off-by: Muhammad Mahad <[email protected]>

gcc/testsuite/rust/rustc/README.md

@@ -0,0 +1,4 @@
+This repository contains test cases from the 
+[rustc test suite](https://github.com/rust-lang/rust/tree/master/tests). The
+conversion of these tests into the DejaGnu format is done by the rustc 
+testsuite adaptor, a tool specifically designed for this purpose.

gcc/testsuite/rust/rustc/rustc.exp

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+# Copyright (C) 2021-2024 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+# 
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+# 
+# You should have received a copy of the GNU General Public License
+# along with GCC; see the file COPYING3.  If not see
+# <http://www.gnu.org/licenses/>.
+
+# Compile tests, no torture testing.
+#
+# These tests raise errors in the front end; torture testing doesn't apply.
+
+# Load support procs.
+load_lib rust-dg.exp
+
+# Initialize `dg'.
+dg-init
+
+# Main loop.
+set saved-dg-do-what-default ${dg-do-what-default}
+
+set dg-do-what-default "compile"
+dg-runtest [lsort [glob -nocomplain $srcdir/$subdir/*.rs]] "" ""
+set dg-do-what-default ${saved-dg-do-what-default}
+
+# All done.
+dg-finish