arm64: Save/restore TPIDR_EL0 in _arch_context_swap

It's not used for anything by the OS but userspace might want
to use it for its own purposes.

Change-Id: Icda4bcd6de9d68596555e81293c8bd075f80a2a4
Reviewed-on: https://review.haiku-os.org/c/haiku/+/7511
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

headers/private/kernel/arch/arm64/arch_thread_types.h

@@ -18,7 +18,7 @@ struct iframe_stack {
 
 
 struct arch_thread {
-	uint64 regs[13]; // x19-x30, sp
+	uint64 regs[14]; // x19-x30, sp, tpidr_el0
 	uint64 fp_regs[8]; // d8-d15
 
 	// used to track interrupts on this thread

src/system/kernel/arch/arm64/arch_asm.S

@@ -253,7 +253,8 @@ FUNCTION(_arch_context_swap):
 	stp x29, x30, [x0], #16
 
 	mov x2, sp
-	str x2, [x0], #8
+	mrs x3, TPIDR_EL0
+	stp  x2,  x3, [x0], #16
 
 	stp  d8,  d9, [x0], #16
 	stp d10, d11, [x0], #16
@@ -268,8 +269,9 @@ FUNCTION(_arch_context_swap):
 	ldp x27, x28, [x1], #16
 	ldp x29, x30, [x1], #16
 
-	ldr x2, [x1], #8
+	ldp  x2,  x3, [x1], #16
 	mov sp, x2
+	msr TPIDR_EL0, x3
 
 	ldp  d8,  d9, [x1], #16
 	ldp d10, d11, [x1], #16