Add verbose logging support to gdb-remote tests

Summary:
gdb-remote tests are not able to use the same logging mechanisms as the rest of our tests, and
currently we get no host logs from them, even though the tests themselves have logging
capability. This commit changes that. When user specifies that he would like to log the
gdb-remote channel (--channel gdb-remote argument to dotest.py), we write detailed logs to the
<TEST_ID>-host.log file, just like we would in the case of regular tests. If this argument is not
specified, we only log the serious messages to stderr, which matches the existing behaviour.

Reviewers: tfiala, tberghammer

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D16858

llvm-svn: 259774