Fix lld detection in standalone compiler-rt.

Summary:
Right now all hwasan tests on Android are silently disabled because they
require "has_lld" and standalone compiler-rt can not (and AFAIK was
never able to) set it.

Reviewers: pcc

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

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D69405