Since v8config.h already defines the related logic for OS detection, it
should be fine to move arch detection also there. The CL also makes
Oilpan use one of those macros to avoid discrepancy between the compiler
and custom macros (the discrepancy occurs for host tools, e.g. mksnapshot).

Bug: chromium:1336222
Change-Id: Ibe7d9116d27c3e816f4778fd4c149d8142d0f7a9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3726208
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81397}