Prepare for |is_posix| switch in the Fuchsia build in yasm/

|is_posix| will be switched to false for Fuchsia, this is a preliminary change.

Bug: 812974
Change-Id: I682e0aca08051f4c930ea7b3435bdf3ca0c91d2b
Reviewed-on: https://chromium-review.googlesource.com/988811
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Fabrice de Gans-Riberi <fdegans@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#548195}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 30efd932c4641a19e91dfe9c23265580e7b0d994