In preparation for kRootRegister support on ia32.

For both descriptors we simply shuffle registers around to remove ebx
from the calling convention.

Possible follow-up work: The ApiCallbackDescriptor could be simplified
by passing call_data (and the Undefined constant) on the stack. This
currently happens in the builtin body.

Drive-by: Minor refactoring in InterpreterPushArgsMode to deobfuscate
the different paths (spread/no-spread). Also use
{Push,Pop}ReturnAddress helpers.

Bug: v8:6666
Change-Id: I25fd738501fff71c038a0745cec04363f90df660
Reviewed-on: https://chromium-review.googlesource.com/1196552Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55554}