nios2: add bits/atomic.h

Without an arch header, nios2 fell back to the non-atomic implementation
in include/atomic.h, so a compare-and-exchange could lose a concurrent
update.  In practice that cost condvar wakeups: tst-cond16 and tst-cond17
spun forever under qemu.

nios2 R1 has no atomic instruction.  The kernel exports __kuser_cmpxchg
at 0x1004 and rewinds the load/store pair when it interrupts it, and
libgcc's nios2 __sync_* functions are built on exactly that helper, so
use the builtins.  There is no 64-bit variant; NPTL and include/atomic.h
only use int- and pointer-sized objects.

Signed-off-by: Ramin Moussavi <lordrasmus@gmail.com>

libc/sysdeps/linux/nios2/bits/atomic.h

@@ -0,0 +1,71 @@
+/* Copyright (C) 2026 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library.  If not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#ifndef _NIOS2_BITS_ATOMIC_H
+#define _NIOS2_BITS_ATOMIC_H
+
+#include <stdint.h>
+
+typedef int8_t atomic8_t;
+typedef uint8_t uatomic8_t;
+typedef int_fast8_t atomic_fast8_t;
+typedef uint_fast8_t uatomic_fast8_t;
+
+typedef int16_t atomic16_t;
+typedef uint16_t uatomic16_t;
+typedef int_fast16_t atomic_fast16_t;
+typedef uint_fast16_t uatomic_fast16_t;
+
+typedef int32_t atomic32_t;
+typedef uint32_t uatomic32_t;
+typedef int_fast32_t atomic_fast32_t;
+typedef uint_fast32_t uatomic_fast32_t;
+
+typedef int64_t atomic64_t;
+typedef uint64_t uatomic64_t;
+typedef int_fast64_t atomic_fast64_t;
+typedef uint_fast64_t uatomic_fast64_t;
+
+typedef intptr_t atomicptr_t;
+typedef uintptr_t uatomicptr_t;
+typedef intmax_t atomic_max_t;
+typedef uintmax_t uatomic_max_t;
+
+/* nios2 is uniprocessor, so a compiler barrier is enough.  */
+#define atomic_full_barrier()	__asm__ __volatile__ ("" ::: "memory")
+#define atomic_read_barrier()	atomic_full_barrier ()
+#define atomic_write_barrier()	atomic_full_barrier ()
+
+/* nios2 R1 has no atomic instruction.  libgcc implements __sync_* through
+   the kernel's __kuser_cmpxchg helper at 0x1004, which the kernel restarts
+   when it interrupts the load/store pair.  */
+#define __arch_compare_and_exchange_val_8_acq(mem, newval, oldval) \
+  __sync_val_compare_and_swap ((mem), (oldval), (newval))
+
+#define __arch_compare_and_exchange_val_16_acq(mem, newval, oldval) \
+  __sync_val_compare_and_swap ((mem), (oldval), (newval))
+
+#define __arch_compare_and_exchange_val_32_acq(mem, newval, oldval) \
+  __sync_val_compare_and_swap ((mem), (oldval), (newval))
+
+/* No 64-bit CAS.  NPTL and include/atomic.h only use int- and
+   pointer-sized objects, so the bysize dispatch never reaches this.  */
+#define __arch_compare_and_exchange_val_64_acq(mem, newval, oldval) \
+  ({ __typeof (*(mem)) __ret = *(mem); abort (); (void) (newval);	\
+     (void) (oldval); __ret; })
+
+#endif /* _NIOS2_BITS_ATOMIC_H */