Convert Linkage to use MachineSignature.

This simplifies the handling of MachineTypes for parameters and returns
used in tests, and overall improves the regularity with which they
are handled in both tests and in CallDescriptor.

R=bmeurer@chromium.org, jarin@chromium.org
BUG=

Review URL: https://codereview.chromium.org/530783002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23638 ce2b1a6d-e550-0410-aec6-3dcde31c8c00