[libFuzzer] Re-enable libFuzzer on i386 Linux and fix test
Re-enable libFuzzer on i386 Linux after it was accidentally
Also disable gc-sections.test on i386 since lld isn't
garbage collecting properly with ASAN on i386.
Reviewed By: morehouse
Subscribers: srhines, mgorny, #sanitizers, llvm-commits
Tags: #sanitizers, #llvm
Differential Revision: https://reviews.llvm.org/D61415