arm64: insn: avoid circular include dependency

Nathan reports that when building with CONFIG_LTO_CLANG_THIN=y, the
build fails due to BUILD_BUG_ON() not being defined before its uss in
<asm/insn.h>.

The problem is that with LTO, we patch READ_ONCE(), and <asm/rwonce.h>
includes <asm/insn.h>, creating a circular include chain:

<linux/build_bug.h>
<linux/compiler.h>
<asm/rwonce.h>
<asm/alternative-macros.h>
<asm/insn.h>
<linux/build-bug.h>

... and so when <asm/insn.h> includes <linux/build_bug.h>, none of the
BUILD_BUG* definitions have happened yet.

To avoid this, let's move AARCH64_INSN_SIZE into a header without any
dependencies, such that it can always be safely included. At the same
time, avoid including <asm/alternative.h> in <asm/insn.h>, which should
no longer be necessary (and doesn't make sense when insn.h is consumed
by userspace).

Reported-by: Nathan Chancellor <[e-mail]>
Signed-off-by: Mark Rutland <[e-mail]>
Cc: Catalin Marinas <[e-mail]>
Cc: Will Deacon <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Fixes: 3e00e39d9dad ("arm64: insn: move AARCH64_INSN_SIZE into <asm/insn.h>")
Signed-off-by: Will Deacon <[e-mail]>

arch/arm64/include/asm/alternative-macros.h

···
 #define __ASM_ALTERNATIVE_MACROS_H
 
 #include <asm/cpucaps.h>
-#include <asm/insn.h>
+#include <asm/insn-def.h>
 
 #define ARM64_CB_PATCH ARM64_NCAPS
 

arch/arm64/include/asm/insn-def.h

···
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#ifndef __ASM_INSN_DEF_H
+#define __ASM_INSN_DEF_H
+
+/* A64 instructions are always 32 bits. */
+#define	AARCH64_INSN_SIZE		4
+
+#endif /* __ASM_INSN_DEF_H */

arch/arm64/include/asm/insn.h

···
 #include <linux/build_bug.h>
 #include <linux/types.h>
 
-#include <asm/alternative.h>
-
-/* A64 instructions are always 32 bits. */
-#define	AARCH64_INSN_SIZE		4
+#include <asm/insn-def.h>
 
 #ifndef __ASSEMBLY__
 /*