Added utility function to get correct signal number from remote platform.

Summary:
This change adds a utility that uses the `kill -l` command to get the
correct signal number. Falls back to using `SBUnixSignals`, and finally
`signal.SIG<signal>` if all else fails.

Reviewers: tberghammer, clayborg, ovyalov

Reviewed By: clayborg, ovyalov

Subscribers: tberghammer, lldb-commits

Differential Revision: http://reviews.llvm.org/D10171

llvm-svn: 238850