When Assembler::nop is in the header, it is considered an inline
function. With GN arg is_component_build=true, the V8_EXPORT_PRIVATE
mark on the class causes it to be exported every time the header is
included. This, in turn, produces a reference to
Register::XRegFromCode.

Register::XRegFromCode is only ever defined as an inlined function, so
that reference is never fulfilled.

Clang can avoid this using the /Fc:dllexportInlines- flag to suppress
the export of Assembler::nop and so avoid generating the reference to
Register::XRegFromCode.

MSVC does not support this flag, so this change suppresses the export
by moving Assembler::nop's definition to the .cc file. This also allows
it to use the inline definition of Register::XRegFromCode.

Bug: v8:8870
Change-Id: I1cd33195677256c9dd06c7047fe84e1b912d3151
Reviewed-on: https://chromium-review.googlesource.com/c/1478216Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59785}