arm64: Use the PAN msr mnemonic rather than .inst

Switch from creating the msr instructions to manage PAN to use the
"msr pan, #1" instruction directly. When this was added clang didn't
have support to assemble the instructions. This appears to have been
added to clang 13 which is sufficiently old enough.

Binutils releases from around the same time appear to have added this
instruction so any modern gcc should also support this instruction.

Sponsored by:	Arm Ltd
Differential Revision:	https://reviews.freebsd.org/D47817

sys/arm64/arm64/machdep.c

@@ -187,19 +187,17 @@ pan_enable(void)
 {
 
 	/*
-	 * The LLVM integrated assembler doesn't understand the PAN
-	 * PSTATE field. Because of this we need to manually create
-	 * the instruction in an asm block. This is equivalent to:
-	 * msr pan, #1
-	 *
 	 * This sets the PAN bit, stopping the kernel from accessing
 	 * memory when userspace can also access it unless the kernel
 	 * uses the userspace load/store instructions.
 	 */
 	if (has_pan) {
 		WRITE_SPECIALREG(sctlr_el1,
 		    READ_SPECIALREG(sctlr_el1) & ~SCTLR_SPAN);
-		__asm __volatile(".inst 0xd500409f | (0x1 << 8)");
+		__asm __volatile(
+		    ".arch_extension pan	\n"
+		    "msr pan, #1		\n"
+		    ".arch_extension nopan	\n");
 	}
 }
 

sys/arm64/include/asm.h

@@ -87,19 +87,25 @@
 	ldr	tmp, =has_pan;			/* Get the addr of has_pan */ \
 	ldr	reg, [tmp];			/* Read it */		\
 	cbz	reg, 997f;			/* If no PAN skip */	\
-	.inst	0xd500409f | (0 << 8);		/* Clear PAN */		\
+	.arch_extension pan;						\
+	msr pan, #0;				/* Disable PAN checks */ \
+	.arch_extension nopan;						\
 	997:
 
 #define	EXIT_USER_ACCESS(reg)						\
 	cbz	reg, 998f;			/* If no PAN skip */	\
-	.inst	0xd500409f | (1 << 8);		/* Set PAN */		\
+	.arch_extension pan;						\
+	msr pan, #1;				/* Enable PAN checks */ \
+	.arch_extension nopan;						\
 	998:
 
 #define	EXIT_USER_ACCESS_CHECK(reg, tmp)				\
 	ldr	tmp, =has_pan;			/* Get the addr of has_pan */ \
 	ldr	reg, [tmp];			/* Read it */		\
 	cbz	reg, 999f;			/* If no PAN skip */	\
-	.inst	0xd500409f | (1 << 8);		/* Set PAN */		\
+	.arch_extension pan;						\
+	msr pan, #1;				/* Enable PAN checks */ \
+	.arch_extension nopan;						\
 	999:
 
 /*