use freestanding headers

arch/x86_64/gdt.c

@@ -1,6 +1,6 @@
 #include <zinnia/percpu.h>
 #include <stddef.h>
-#include <string.h>
+#include <zinnia/string.h>
 #include <x86_64/asm.h>
 #include <x86_64/defs.h>
 #include <x86_64/gdt.h>

arch/x86_64/include/bits/types.h

@@ -3,4 +3,3 @@
 #include <stdint.h>
 
 typedef uint64_t phys_t;
-typedef uint64_t virt_t;