[libFuzzer] Disable i386 on non-Linux platforms

Summary: Disable i386 on non-Linux platforms since it is unwanted and broken on Windows.

Reviewers: morehouse, rnk

Reviewed By: morehouse

Subscribers: mgorny, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D61354

llvm-svn: 359641