MIPS: Make concrete classes for individual call descriptors. - external

Port r23639 (e5a2758)

Original commit message:
The ic-convention classes that hold register specifications are merged into these new call descriptor classes, which should represent a final home for that information.

BUG=
R=machenbach@chromium.org

Review URL: https://codereview.chromium.org/535753003

Patch from Balazs Kilvady <balazs.kilvady@imgtec.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23667 ce2b1a6d-e550-0410-aec6-3dcde31c8c00