Check the host platform to determine logging channel to test in TestRegisters.py

Checks the host platform to determine logging channel to test in
TestRegisters.py as the logging runs host side.

Test Plan:
./dotest.py $DOTEST_OPTS -v -t -p TestRegisters.py
This passes mac -> linux as it's logging the correct channel.

Differential Revision: http://reviews.llvm.org/D9755

llvm-svn: 237271