Fix up lldb/llgs to use proper signals on Linux/*BSD.

ProcessGDBRemote now overrides the unix signal provider.  It
uses the target architecture as finally resolved after connection
to determine the most specific UnixSignals that make sense for
the platform.  This is needed so that the lldb client side can
interpret the llgs signals according to the target architecture.

This fixes a bug where lldb on Linux was interpreting things like
a true SIGSTOP (0x13 on Linux) as a SIGCONT (0x13 on other Unix
flavors), causing all kinds of logic to misfire with regards
to signal handling on the lldb side.

This likely requires cmake and autoconf/make for certain variants
to be adjusted (e.g. FreeBSD cmake will need LinuxSignals now,
same for MacOSX cmake).