GCN, nvptx: Don't default-disable libstdc++ build

In addition to making libstdc++ itself available, this, via enabling
'build-gcc/*/libstdc++-v3/scripts/testsuite_flags', in particular also makes
the standard C++ headers available to 'make check-gcc-c++'.  With that, there
are a lot of FAIL/UNRESOLVED -> PASS progressions, where we previously ran
into, for example:

    FAIL: g++.dg/coroutines/co-await-syntax-00-needs-expr.C  (test for errors, line 6)
    FAIL: g++.dg/coroutines/co-await-syntax-00-needs-expr.C (test for excess errors)
    Excess errors:
    [...]/gcc/testsuite/g++.dg/coroutines/coro.h:132:10: fatal error: cstdlib: No such file or directory

Similarly, there are a lot of FAIL/UNRESOLVED -> UNSUPPORTED "progressions" due
to 'sorry, unimplemented: exception handling not supported'.

The 'make check-target-libstdc++-v3' results don't look too bad, either.

This also reverts Subversion r221362
(Git commit d94fae044d) "No libstdc++ for nvptx",
and commit 2f4f3c0e9345805160ecacd6de527b519a8c9206 "No libstdc++ for GCN".

With libstdc++ now available, libgrust gets enabled, which we in turn again
have to disable, for 'sorry, unimplemented: exception handling not supported'
reasons.

	PR target/92713
	PR target/101544
	* configure.ac [GCN, nvptx] (noconfigdirs): Don't add
	'target-libstdc++-v3'.  Add 'target-libgrust'.
	* configure: Regenerate.
	gcc/
	* config/gcn/gcn.h (LIBSTDCXX): Don't set.
	* config/nvptx/nvptx.h (LIBSTDCXX): Likewise.

configure

@@ -3431,10 +3431,6 @@ if test "${ENABLE_LIBSTDCXX}" = "default" ; then
       # VxWorks uses the Dinkumware C++ library.
       noconfigdirs="$noconfigdirs target-libstdc++-v3"
       ;;
-    amdgcn*-*-*)
-      # Not ported/fails to build when using newlib.
-      noconfigdirs="$noconfigdirs target-libstdc++-v3"
-      ;;
     arm*-wince-pe*)
       # the C++ libraries don't build on top of CE's C libraries
       noconfigdirs="$noconfigdirs target-libstdc++-v3"
@@ -4148,7 +4144,7 @@ case "${target}" in
     noconfigdirs="$noconfigdirs gprof"
     ;;
   nvptx*-*-*)
-    noconfigdirs="$noconfigdirs target-libssp target-libstdc++-v3"
+    noconfigdirs="$noconfigdirs target-libssp"
     ;;
   sh-*-*)
     case "${target}" in
@@ -10395,6 +10391,16 @@ case ,${enable_languages}, in
         # Disable target libgrust if we're not building target libstdc++.
         noconfigdirs="$noconfigdirs target-libgrust"
         ;;
+      *)
+	case "${target}" in
+	  amdgcn*-*-* \
+	  | nvptx*-*-* )
+	    # Build fails in 'libgrust/libproc_macro_internal/':
+	    #     sorry, unimplemented: exception handling not supported
+	    noconfigdirs="$noconfigdirs target-libgrust"
+	    ;;
+	esac
+	;;
     esac
     ;;
 esac

configure.ac

@@ -657,10 +657,6 @@ if test "${ENABLE_LIBSTDCXX}" = "default" ; then
       # VxWorks uses the Dinkumware C++ library.
       noconfigdirs="$noconfigdirs target-libstdc++-v3"
       ;;
-    amdgcn*-*-*)
-      # Not ported/fails to build when using newlib.
-      noconfigdirs="$noconfigdirs target-libstdc++-v3"
-      ;;
     arm*-wince-pe*)
       # the C++ libraries don't build on top of CE's C libraries
       noconfigdirs="$noconfigdirs target-libstdc++-v3"
@@ -1365,7 +1361,7 @@ case "${target}" in
     noconfigdirs="$noconfigdirs gprof"
     ;;
   nvptx*-*-*)
-    noconfigdirs="$noconfigdirs target-libssp target-libstdc++-v3"
+    noconfigdirs="$noconfigdirs target-libssp"
     ;;
   sh-*-*)
     case "${target}" in
@@ -2696,6 +2692,16 @@ case ,${enable_languages}, in
         # Disable target libgrust if we're not building target libstdc++.
         noconfigdirs="$noconfigdirs target-libgrust"
         ;;
+      *)
+	case "${target}" in
+	  amdgcn*-*-* \
+	  | nvptx*-*-* )
+	    # Build fails in 'libgrust/libproc_macro_internal/':
+	    #     sorry, unimplemented: exception handling not supported
+	    noconfigdirs="$noconfigdirs target-libgrust"
+	    ;;
+	esac
+	;;
     esac
     ;;
 esac

gcc/config/gcn/gcn.h

@@ -867,7 +867,3 @@ enum gcn_builtin_codes
       || M == V2SFmode || M == V2DImode || M == V2DFmode) \
    ? 2 \
    : 1)
-
-/* The C++ front end insists to link against libstdc++ -- which we don't build.
-   Tell it to instead link against the innocuous libgcc.  */
-#define LIBSTDCXX "gcc"

gcc/config/nvptx/nvptx.h

@@ -379,10 +379,6 @@ struct GTY(()) machine_function
 /* See 'libgcc/config/nvptx/crt0.c' for wrapping of 'main'.  */
 #define HAS_INIT_SECTION
 
-/* The C++ front end insists to link against libstdc++ -- which we don't build.
-   Tell it to instead link against the innocuous libgcc.  */
-#define LIBSTDCXX "gcc"
-
 /* The default doesn't fly ('internal compiler error: in simplify_subreg' when
    'dw2_assemble_integer' -> 'assemble_integer' attempts to simplify
    '(minus:DI (symbol_ref:DI ("$LEHB0")) (symbol_ref:DI ("$LFB3")))', for