Move string functions/utils from HAL to std/string

kernel/hal/x86_64/gdt.c

@@ -3,6 +3,7 @@
 #include "compiler/attr.h"
 #include "hal/hal.h"
 #include "gdt.h"
+#include "std/string.h"
 
 #define GDT_PRESENT 0x80
 #define GDT_TSS     0x89
@@ -43,7 +44,7 @@ void gdt_setentry(GdtEntry *ent, uint32_t base, uint32_t limit, uint8_t acc, uin
 }
 
 void gdt_init(void) {
-  hal_memset(&tss, 0, sizeof(tss));
+  memset(&tss, 0, sizeof(tss));
   tss.iopb_off = sizeof(tss);
 
   tss.rsp0 = (uint64_t)(kernelstack + sizeof(kernelstack));