xtensa: enable context tracking

Put user exit context tracking call on the common kernel entry/exit path
(function calls are impossible at earlier kernel entry stages because
PS.EXCM is not cleared yet). Put user entry context tracking call on the
user exit path. Syscalls go through this common code too, so nothing
specific needs to be done for them.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

Documentation/features/time/context-tracking/arch-support.txt

@@ -27,5 +27,5 @@
     |       sparc: |  ok  |
     |          um: | TODO |
     |         x86: |  ok  |
-    |      xtensa: | TODO |
+    |      xtensa: |  ok  |
     -----------------------