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/lib/usercopy.S

@@ -222,8 +222,8 @@ __xtensa_copy_user:
 	loopnez	a7, .Loop2done
 #else /* !XCHAL_HAVE_LOOPS */
 	beqz	a7, .Loop2done
-	slli	a10, a7, 4
-	add	a10, a10, a3	# a10 = end of last 16B source chunk
+	slli	a12, a7, 4
+	add	a12, a12, a3	# a12 = end of last 16B source chunk
 #endif /* !XCHAL_HAVE_LOOPS */
 .Loop2:
 	EX(l32i, a7, a3,  4, l_fixup)
@@ -241,7 +241,7 @@ __xtensa_copy_user:
 	EX(s32i, a9, a5, 12, s_fixup)
 	addi	a5, a5, 16
 #if !XCHAL_HAVE_LOOPS
-	blt	a3, a10, .Loop2
+	blt	a3, a12, .Loop2
 #endif /* !XCHAL_HAVE_LOOPS */
 .Loop2done:
 	bbci.l	a4, 3, .L12