[gcc r16-9566] build: Add stdatomic.h to USER_H on OpenBSD

https://gcc.gnu.org/g:980bfacc116703fb994f780a6a6fb7a164fd6fc0

commit r16-9566-g980bfacc116703fb994f780a6a6fb7a164fd6fc0
Author: Pietro Monteiro <[email protected]>
Date:   Thu Aug 20 11:54:57 2026 -0400

    build: Add stdatomic.h to USER_H on OpenBSD
    
    Otherwise libatomic build fails with:
    ../../../gcc/libatomic/fence.c:27:10: fatal error: stdatomic.h: No such file or directory
       27 | #include <stdatomic.h>
          |          ^~~~~~~~~~~~~
    compilation terminated.
    ../../../gcc/libatomic/flag.c:27:10: fatal error: stdatomic.h: No such file or directory
       27 | #include <stdatomic.h>
    
    gcc/ChangeLog:
    
            * config/t-openbsd (USER_H): Add $(srcdir)/ginclude/stdatomic.h.
    
    Signed-off-by: Pietro Monteiro <[email protected]>
    (cherry picked from commit d32c1f0238f33b0f6a3ef1b5b228678a92585094)

Diff:
---
 gcc/config/t-openbsd | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/gcc/config/t-openbsd b/gcc/config/t-openbsd
index 3b625d62b30d..eba06fdd5c4c 100644
--- a/gcc/config/t-openbsd
+++ b/gcc/config/t-openbsd
@@ -1,5 +1,6 @@
-# We don't need GCC's own include files.
-USER_H = $(EXTRA_HEADERS)
+# We need stdatomic.h from GCC's include files.
+USER_H = $(srcdir)/ginclude/stdatomic.h \
+	$(EXTRA_HEADERS)
 
 # OpenBSD-specific D support.
 openbsd-d.o: $(srcdir)/config/openbsd-d.cc