sys/pcpu.h: remove a workaround for a fixed ld bug

The workaround was incorrectly documented as having something to do with
set_pcpu section's progbits, but in fact it was for incorrect placement
of __start_set_pcpu because of the bug in ld.
The bug was fixed in r210245, see commit message for details.

A side-effect of the workaround was that a zero-size set_pcpu section was
produced for modules, source code of which included pcpu.h but didn't
actually define any dynamic per-cpu variables.
This commit should remove the side-effect.

The same workaround is present sys/net/vnet.h, has an analogous side-effect
and can be removed as well.

An UPDATING entry that warns about a need for recent ld is following.

MFC after:	1 month

sys/sys/pcpu.h

@@ -44,24 +44,10 @@
 
 /*
  * Define a set for pcpu data.
- * 
- * We don't use SET_DECLARE because it defines the set as 'a' when we
- * want 'aw'.  gcc considers uninitialized data in a separate section
- * writable, and there is no generic zero initializer that works for
- * structs and scalars.
  */
 extern uintptr_t *__start_set_pcpu;
 extern uintptr_t *__stop_set_pcpu;
 
-__asm__(
-#ifdef __arm__
-	".section set_pcpu, \"aw\", %progbits\n"
-#else
-	".section set_pcpu, \"aw\", @progbits\n"
-#endif
-	"\t.p2align " __XSTRING(CACHE_LINE_SHIFT) "\n"
-	"\t.previous");
-
 /*
  * Array of dynamic pcpu base offsets.  Indexed by id.
  */