kernel/x86_64: Store FPU state from int_bottom_user in arch_thread always.

The XSAVE area can be quite large (1-2KB+), so rather than always
putting it on the kernel stack, just use the arch_thread FPU state.

Change-Id: Ie64c3ea3ca52cd8a4425ae1da69792588a69832a
Reviewed-on: https://review.haiku-os.org/c/haiku/+/9090
Reviewed-by: Jérôme Duval <[email protected]>
Reviewed-by: waddlesplash <[email protected]>
Tested-by: Commit checker robot <[email protected]>

headers/private/kernel/arch/x86/arch_thread_types.h

@@ -63,7 +63,7 @@ struct arch_thread {
 	// have enough space for all the registers, at least 2560 bytes according
 	// to Intel Architecture Instruction Set Extensions Programming Reference,
 	// Section 3.2.4, table 3-8
-	uint8			fpu_state[2560] _ALIGNED(64);
+	uint8			user_fpu_state[2560] _ALIGNED(64);
 #endif
 
 	addr_t			GetFramePointer() const;