libstdc++: Fix up bits/std.cc symlink installation

>       * src/c++23/Makefile.am (libstdc++.modules.json): Rewrite
>       without abspath make function.
>       (stamp-modules-bits): Likewise.
>       * src/c++23/Makefile.in: Rebuilt.

This change broke various module tests (and got backported even to 15 :( ).
I'm using objdir as a subdirectory of the gcc tree, so ../configure ...

> @@ -57,7 +57,9 @@ std.compat.cc: std.compat.cc.in std-clib.cc.in
>  # Also put the interface units in the build-includes bits directory.
>  stamp-modules-bits: $(includebits_DATA)
>       @-mkdir -p $(top_builddir)/include/bits
> -     -cd $(top_builddir)/include/bits && $(LN_S) $(abspath $?) . 2>/dev/null
> +     -for f in $(includebits_DATA); do \
> +       $(LN_S) $(abs_srcdir)/$$f $(top_builddir)/include/bits; \
> +     done 2>/dev/null
>       @$(STAMP) $@
>
>  all-local: stamp-module-manifest stamp-modules-bits

Before this change, the
/home/jakub/src/gcc/obj38/x86_64-pc-linux-gnu/libstdc++-v3/include/bits
std.cc symlink used to point to
/home/jakub/src/gcc/obj38/x86_64-pc-linux-gnu/libstdc++-v3/src/c++23/std.cc
Now it points to
/home/jakub/src/gcc/obj40/x86_64-pc-linux-g~-v3/../../../libstdc++-v3/src/c++23/std.cc
which doesn't exist, the source directory only has std.cc.in file from which
the std.cc file is generated in the build directory.

The following patch fixes it by using abs_builddir instead of abs_srcdir.

Fixes up
-FAIL: g++.dg/modules/compile-std1.C -std=c++29 (test for excess errors)
-FAIL: g++.dg/modules/compile-std1.C -std=c++29 module-cmi std (gcm.cache/std.gcm)
-FAIL: g++.dg/modules/compile-std1.C -std=c++29 module-cmi std.compat (gcm.cache/std.compat.gcm)
-FAIL: g++.dg/plugin/std-module-exports-c++20.C -fplugin=./std_module_exports_plugin.so (test for excess errors)
-FAIL: g++.dg/plugin/std-module-exports-c++23.C -fplugin=./std_module_exports_plugin.so (test for excess errors)
-FAIL: g++.dg/plugin/std-module-exports-c++26.C -fplugin=./std_module_exports_plugin.so (test for excess errors)

2026-06-25  Jakub Jelinek  <jakub@redhat.com>

	* src/c++23/Makefile.am (stamp-modules-bits): Use abs_builddir instead
	of abs_srcdir.
	* src/c++23/Makefile.in: Regenerate.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
(cherry picked from commit 9274edef95ab1b74b82f4a9cf14baa9004e03c0d)