Fix linker option for -fprofile-arcs -ftest-coverage

Summary:
Linux toolchain accidentally added "-u__llvm_runtime_variable" when "-fprofile-arcs -ftest-coverage", this is not added when "--coverage" option is used.
Using "-u__llvm_runtime_variable" generates an empty default.profraw file while an application built with  "-fprofile-arcs -ftest-coverage" is running. 

Reviewers: calixte, marco-c, sylvestre.ledru

Reviewed By: marco-c

Subscribers: vsk, cfe-commits

Differential Revision: https://reviews.llvm.org/D54195

llvm-svn: 347677