toplevel: Build stage1/stage2/stageprofile libstdc++ with --disable-libstdcxx-pch...

toplevel: Build stage1/stage2/stageprofile libstdc++ with --disable-libstdcxx-pch when bootstrapping

The following patch saves ~ 2.4GiB of disk space in x86_64-linux
bootstrapped object directory:
find obj80 -name \*.gch -a -type f | xargs du -shc | grep total
3.7G    total
find obj82 -name \*.gch -a -type f | xargs du -shc | grep total
1.3G    total
and ~ 800MiB for i686-linux:
find obj81 -name \*.gch -a -type f | xargs du -shc | grep total
1.2G    total
find obj83 -name \*.gch -a -type f | xargs du -shc | grep total
409M    total
by disabling PCH in stage1/stage2/stageprofile builds, so only
building it in stage3/stagefeedback etc.
I think in stage1/stage2 it is a pure waste of bootstrap time and disk
space, for profiledbootstrap I'd say PCH isn't used commonly enough
in the wild that it is worth training GCC on that (but if you disagree,
I can surely take out that single line in there).

2026-02-04  Jakub Jelinek  <jakub@redhat.com>

	* Makefile.tpl (STAGE1_CONFIGURE_FLAGS, STAGE2_CONFIGURE_FLAGS,
	STAGEprofile_CONFIGURE_FLAGS): Append --disable-libstdcxx-pch if
	target-libstdc++-v3-bootstrap.
	* Makefile.in: Regenerate.