Note that according to the System V ABI for AMD64, rbx must be
preserved across calls. We actually crash with clang in the x64
assembler tests without that fix, we were lucky with GCC.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/144313017

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@19178 ce2b1a6d-e550-0410-aec6-3dcde31c8c00