[compiler-rt][tests] Propagate COMPILER_RT_UNITTEST_LINK_FLAGS

`COMPILER_RT_UNITTEST_LINK_FLAGS` is dropped in many places, unlike
`COMPILER_RT_UNITTEST_CFLAGS`. This patch attempts to remove that
inconsistency.

Previously reviewed as part of D58951.

Reviewers: sfertile, peter.smith, pzheng, phosek, Hahnfeld, nemanjai, jasonliu

Reviewed By: sfertile

Subscribers: jsji, kubamracek, dberris, mgorny, delcypher, jdoerfert, #sanitizers, llvm-commits

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D60143

llvm-svn: 359733