Linkage-related methods were declared in wasm-compiler.h and
implemented in wasm-linkage.cc. This required all users of e.g. wasm
call descriptors to include the whole wasm compiler header. Also, some
wasm linkage information is independent of turbofan and also used
outside of the compiler directory.

This CL splits off wasm-linkage.h (with minimal includes) and puts it
in src/wasm. This allows to use that information without including
compiler headers (will clean up several uses in follow-up CLs).

R=mstarzinger@chromium.org, titzer@chromium.org

Bug: v8:7570
Change-Id: Ifcae70b4ea7932cda30953b325c2b87c4176c598
Reviewed-on: https://chromium-review.googlesource.com/1013701Reviewed-by: Ben Titzer <titzer@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52849}