x86_64: implement proper user/kernel %gs base switching

the %gs base address is now always set to the current cpu block while
in kernel-mode, and is switched back to the userspace %gs base
when returning to user-mode.

arch/x86_64/tss.c

@@ -1,5 +1,3 @@
-#include "arch/msr.h"
-
 #include <arch/gdt.h>
 #include <arch/tss.h>
 #include <kernel/libc/string.h>
@@ -22,9 +20,6 @@ void tss_init(struct tss *tss, struct tss_ptr *ptr)
 void tss_load(struct tss *tss)
 {
 	tss_flush(TSS_GDT_INDEX);
-
-	uintptr_t kernel_gs_base_reg = 0xC0000102;
-	wrmsr(kernel_gs_base_reg, (uintptr_t)tss);
 }
 
 virt_addr_t tss_get_kstack(struct tss *tss)