DRY out PIT and make one that fires only once, get nsectors from linker script

linker.ld

@@ -8,6 +8,8 @@ SECTIONS
     . = 0x7c00;
     .text :
     {
+        __start = .;
+
         /*
         We are going to stuff everything
         into a text segment for now, including data.
@@ -31,18 +33,20 @@ SECTIONS
         *(.stage2)
 
         /*
-        TODO get this working.
-
-        Number of sectors in stage 2. Used by the `int 13`.
+        Number of sectors in stage 2. Used by the `int 13` to load it from disk.
 
         The value gets put into memory as the very last thing
         in the `.stage` section if it exists.
 
         We must put it *before* the final `. = ALIGN(512)`,
         or else it would fall out of the loaded memory.
+
+        This must be absolute, or else it would get converted
+        to the actual address relative to this section (7c00 + ...)
+        and linking would fail with "Relocation truncated to fit"
+        because we are trying to put that into al for the int 13.
          */
-        __stage2_size = .;
-        /*BYTE((ALIGN(.) / 512) - 1);*/
+        __stage2_nsectors = ABSOLUTE((. - __start) / 512);
 
         /* Ensure that the generated image is a multiple of 512 bytes long. */
         . = ALIGN(512);