Make sure to include standard headers that are tightly bound
to the compiler, such as stdatomic.h

Index: gcc/config/t-openbsd
--- gcc/config/t-openbsd.orig
+++ gcc/config/t-openbsd
@@ -1,2 +1,6 @@
 # We don't need GCC's own include files.
-USER_H = $(EXTRA_HEADERS)
+USER_H = $(srcdir)/ginclude/stdfix.h \
+	 $(srcdir)/ginclude/stdnoreturn.h \
+	 $(srcdir)/ginclude/stdalign.h \
+	 $(srcdir)/ginclude/stdatomic.h \
+	 $(EXTRA_HEADERS)