[test] Fix compiler-rt/test/profile/coverage_emptylines.cpp if the build directory is under /tmp

llvm-cov -path-equivalence=/tmp,... is used by some checked-in coverage mapping
files where the original filename is under /tmp. If the test itself produces the
coverage mapping file, there is no need for /tmp.

For coverage_emptylines.cpp: the source filename is under the build directory.
If the build directory is under /tmp, the path mapping will make
llvm-cov fail to find the file.