xtensa: Remove 64 bits atomic builtins stubs

The stubs were provided by commit 725aea873261 ("xtensa: enable KCSAN")
to make linker happy allthought they are not meant to be used at all.

KCSAN core has been fixed to not require them anymore on
32 bits architectures.

Then they can be removed.

Signed-off-by: Christophe Leroy <[e-mail]>
Acked-by: Max Filippov <[e-mail]>
Acked-by: Marco Elver <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>
Link: https://msgid.link/a6834980[e-mail]

arch/xtensa/lib/Makefile

···
 	   divsi3.o udivsi3.o modsi3.o umodsi3.o mulsi3.o umulsidi3.o \
 	   usercopy.o strncpy_user.o strnlen_user.o
 lib-$(CONFIG_PCI) += pci-auto.o
-lib-$(CONFIG_KCSAN) += kcsan-stubs.o
-KCSAN_SANITIZE_kcsan-stubs.o := n

arch/xtensa/lib/kcsan-stubs.c

···
-// SPDX-License-Identifier: GPL-2.0
-
-#include <linux/bug.h>
-#include <linux/types.h>
-
-void __atomic_store_8(volatile void *p, u64 v, int i)
-{
-	BUG();
-}
-
-u64 __atomic_load_8(const volatile void *p, int i)
-{
-	BUG();
-}
-
-u64 __atomic_exchange_8(volatile void *p, u64 v, int i)
-{
-	BUG();
-}
-
-bool __atomic_compare_exchange_8(volatile void *p1, void *p2, u64 v, bool b, int i1, int i2)
-{
-	BUG();
-}
-
-u64 __atomic_fetch_add_8(volatile void *p, u64 v, int i)
-{
-	BUG();
-}
-
-u64 __atomic_fetch_sub_8(volatile void *p, u64 v, int i)
-{
-	BUG();
-}
-
-u64 __atomic_fetch_and_8(volatile void *p, u64 v, int i)
-{
-	BUG();
-}
-
-u64 __atomic_fetch_or_8(volatile void *p, u64 v, int i)
-{
-	BUG();
-}
-
-u64 __atomic_fetch_xor_8(volatile void *p, u64 v, int i)
-{
-	BUG();
-}
-
-u64 __atomic_fetch_nand_8(volatile void *p, u64 v, int i)
-{
-	BUG();
-}