Corrected register calling syntax in assembler-s390.cc and
test-platform.cc.

Generate_CEntry in builtins-s390.cc expects return buffer to be
preserved in r2, but when built with clang r2 isn't preserved, which breaks
300+ tests. It is fixed by writing r2's value into r8 (preserved)
and loading the value back to r2 after the operation.

Change-Id: I184f0111944b6ad8c0ccc8b97407d702dd97d9d8
Reviewed-on: https://chromium-review.googlesource.com/1204530Reviewed-by: Junliang Yan <jyan@ca.ibm.com>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55631}