target-xtensa: add missing window check for entry

Entry opcode needs to check if moving to new register frame would cause
register window overflow. Entry used in function prologue never
overflows because preceding windowed call* opcode writes return address
to the target register window frame, causing overflow exceptions at the
point of call. But when a sequence of entry opcodes is used for register
window spilling there may not be a call or other opcode that would cause
window check between entries and they would not raise overflow exception
themselves resulting in data corruption.

Cc: qemu-stable@nongnu.org
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
target-xtensa: add entry overflow test

Check that entry instruction raises window overflow exception when
PS.CALLINC points to live registers.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Merge remote-tracking branch 'remotes/xtensa/tags/20141110-xtensa' into staging

Xtensa fixes for 2.2:
- fix entry opcode register window checking and add unit test.

# gpg: Signature made Mon 10 Nov 2014 15:01:47 GMT using RSA key ID F83FA044
# gpg: Good signature from "Max Filippov <max.filippov@cogentembedded.com>"
# gpg:                 aka "Max Filippov <jcmvbkbc@gmail.com>"

* remotes/xtensa/tags/20141110-xtensa:
  target-xtensa: add entry overflow test
  target-xtensa: add missing window check for entry

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>