Restore signal delivery to the inferior on FreeBSD

This was broken in the threaded inferior implementation for FreeBSD
(r196787) and caused FreeBSD to resume always with no signal.
Converted TestConstVariables expected failure to @expectedFailureClang.

TestConstVariables.py was disabled on Linux and marked as
unconditional expected failure. This change removes the Linux disable
and makes the expected failure conditional on using clang. The test
runs fine on gcc-built lldb.