This CL allows us to distinguish between the host- and target OS. The
host OS is defined by V8_OS_ macros (e.g. V8_OS_WIN). The target OS is
defined by V8_TARGET_OS_ macros (e.g. V8_TARGET_OS_WIN).

V8_TARGET_OS_ macros are defined by gn, based on the `target_os` gn
variable. If a V8_TARGET_OS_ is set, we also define V8_HAVE_TARGET_OS
(this determines fall-back behavior in V8; if it is not defined, we set
V8_TARGET_OS_ to equal the equivalent V8_OS_ define).

Besides adding the defines, this CL also adds logic to consider the
target OS in codegen. Specifically, x64 builds now look at the
V8_TARGET_OS_WIN define instead of V8_OS_WIN or _WIN64. This
effectively makes cross-compilation to x64 Windows in mksnapshot
possible.

In future work, we could add similar support for cross-compiling to
other platforms such as ia32 Windows.

Bug: v8:9736,chromium:803591
Change-Id: I689f3de8c206b743c4bef703f5ade0bba32ce995
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1809374Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63892}