[libFuzzer] Re-enable libFuzzer on i386 Linux and fix test

Summary:
Re-enable libFuzzer on i386 Linux after it was accidentally
disabled.

Also disable gc-sections.test on i386 since lld isn't
garbage collecting properly with ASAN on i386.

Reviewers: morehouse

Reviewed By: morehouse

Subscribers: srhines, mgorny, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D61415

llvm-svn: 359802