This is the first step in cutting the Gordian linkage/linkage-impl knot.

This basically changes the axis along which we organize call descriptor
building logic from having platform-specific files dedicated to all call
descriptor types to having call-descriptor-type-specific files that have

The next step is to factor the JS, code stub, and runtime call descriptors
similarly, dumping them into:
  compiler/js-linkage.cc
  compiler/runtime-linkage.cc
  compiler/code-stub-linkage.cc

 or, alternatively, all of them just into compiler/js-linkage.cc.

This also anticipates a wasm-linkage.cc file in the future.

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

Review URL: https://codereview.chromium.org/1266603002

Cr-Commit-Position: refs/heads/master@{#29931}