arm64: lse: Fix LSE atomics with LLVM

Commit e0d5896b ("arm64: lse: fix LSE atomics with LLVM's integrated
assembler") broke the build when clang is used in connjunction with the
binutils assembler ("-no-integrated-as"). This happens because
__LSE_PREAMBLE is defined as ".arch armv8-a+lse", which overrides the
version of the CPU architecture passed via the "-march" paramter to gas:

$ aarch64-none-linux-gnu-as -EL -I ./arch/arm64/include
                                -I ./arch/arm64/include/generated
                                -I ./include -I ./include
                                -I ./arch/arm64/include/uapi
                                -I ./arch/arm64/include/generated/uapi
                                -I ./include/uapi -I ./include/generated/uapi
                                -I ./init -I ./init
                                -march=armv8.3-a -o init/do_mounts.o
                                /tmp/do_mounts-d7992a.s
/tmp/do_mounts-d7992a.s: Assembler messages:
/tmp/do_mounts-d7992a.s:1959: Error: selected processor does not support `autiasp'
/tmp/do_mounts-d7992a.s:2021: Error: selected processor does not support `paciasp'
/tmp/do_mounts-d7992a.s:2157: Error: selected processor does not support `autiasp'
/tmp/do_mounts-d7992a.s:2175: Error: selected processor does not support `paciasp'
/tmp/do_mounts-d7992a.s:2494: Error: selected processor does not support `autiasp'

Fix the issue by replacing ".arch armv8-a+lse" with ".arch_extension lse".
Sami confirms that the clang integrated assembler does now support the
'.arch_extension' directive, so this change will be fine even for LTO
builds in future.

Fixes: e0d5896b ("arm64: lse: fix LSE atomics with LLVM's integrated assembler")
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Reported-by: Amit Kachhap <Amit.Kachhap@arm.com>
Tested-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/include/asm/lse.h

@@ -6,7 +6,7 @@
 
 #ifdef CONFIG_ARM64_LSE_ATOMICS
 
-#define __LSE_PREAMBLE	".arch armv8-a+lse\n"
+#define __LSE_PREAMBLE	".arch_extension lse\n"
 
 #include <linux/compiler_types.h>
 #include <linux/export.h>