xtensa: fixes for configs without loop option

Build-time fixes:
- make lbeg/lend/lcount save/restore conditional on kernel entry;
- don't clear lcount in platform_restart functions unconditionally.

Run-time fixes:
- use correct end of range register in __endla paired with __loopt, not
  the unused temporary register. This fixes .bss zero-initialization.
  Update comments in asmmacro.h;
- don't clobber a10 in the usercopy that leads to access to unmapped
  memory.

Cc: <stable@vger.kernel.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/platforms/xtfpga/setup.c

@@ -63,7 +63,9 @@ void platform_restart(void)
 #if XCHAL_NUM_IBREAK > 0
 			      "wsr	a2, ibreakenable\n\t"
 #endif
+#if XCHAL_HAVE_LOOPS
 			      "wsr	a2, lcount\n\t"
+#endif
 			      "movi	a2, 0x1f\n\t"
 			      "wsr	a2, ps\n\t"
 			      "isync\n\t"