[metal] Fix bug: do not initialize IDT & TSS if unneeded (#654)

libc/intrin/interrupts.S should not be linked in unless
an IDT (or TSS) is explicitly requested somewhere (i.e.
it should probably not be a mandatory module).

ape/ape.lds

@@ -569,8 +569,6 @@ HIDDEN(v_ape_realsectors =
 HIDDEN(v_ape_realpages = v_ape_realsectors / (4096 / 512));
 HIDDEN(v_ape_highsectors =
            (ROUNDUP(RVA(_edata), 512) / 512) - v_ape_realsectors);
-PROVIDE_HIDDEN(_tss = 0);
-PROVIDE_HIDDEN(_tss_end = 0);
 TSSDESCSTUB2(_tss, _tss, _tss_end ? _tss_end - _tss - 1 : 0);
 #endif
 

ape/macros.internal.h

@@ -182,6 +182,14 @@
 
 //	Task State Segment Descriptor Entries.
 .macro	.tssdescstub name:req
+ .ifndef \name
+  .weak	\name
+  .set	\name,0
+ .endif
+ .ifndef \name\()_end
+  .weak	\name\()_end
+  .set	\name\()_end,0
+ .endif
  .stub	\name\()_desc_ent0,quad
  .stub	\name\()_desc_ent1,quad
 .endm