detatch binutils usage from bootstrap in gcc variants (#18116)

var/spack/repos/builtin/packages/gcc/package.py

@@ -95,6 +95,9 @@ class Gcc(AutotoolsPackage, GNUMirrorPackage):
     variant('nvptx',
             default=False,
             description='Target nvptx offloading to NVIDIA GPUs')
+    variant('bootstrap',
+            default=False,
+            description='add --enable-bootstrap flag for stage3 build')
 
     depends_on('flex', type='build', when='@master')
 
@@ -468,11 +471,15 @@ def configure_args(self):
         if spec.satisfies('+binutils'):
             binutils = spec['binutils'].prefix.bin
             options.extend([
-                '--with-sysroot=/',
                 '--with-gnu-ld',
                 '--with-ld=' + binutils.ld,
                 '--with-gnu-as',
                 '--with-as=' + binutils.join('as'),
+            ])
+
+        # enable_bootstrap
+        if spec.satisfies('+bootstrap'):
+            options.extend([
                 '--enable-bootstrap',
             ])