Enable Linux distribution in vendor portion of host triple.

This change does the following:

* enables building lldb-gdbserver on linux_x86-64 platforms.

  Note - it builds but it has several run-time issues where many gdb
  remote protocol features are not properly implemented yet. I'm
  working on these one at a time.

* lldb-gdbserver: does not enable the eLaunchFlagDebug launch flag on
  Linux. Currently the POSIX launch routine will assert if that flag
  is passed in, presumably because that launch mode is not yet
  available.  This prevents lldb-gdbserver from asserting the moment
  it launches the debuggee process.

* Adds ConstString& Host::GetDistributionId ()

  This method is defined to return an empty result on all platforms
  except for Linux.  On Linux, it makes one attempt to execute
  'lsb_release -i' (both /usr/bin/lsb_release, where it appears
  on ubuntu, and /bin/lsb_release, where it appears on fedora
  if the redhat-lsb package is installed).  If lsb_release is not
  found in either of those locations, or if 'lsb_release -i' does
  not return the first line starting with "Distributor ID:\t",
  then the distribution id is empty.  The method will lower-case
  the id and replace whitespace with underscores.

* Modify Host::GetArchitecture () so that linux replaces an unknown
  vendor portion with the results of GetDistributionId () if that
  is non-empty.  This shows up now in qHostInfo remote packet
  responses and on the lldb host side.  Tested with ubuntu and
  fedora (the latter both with the default of not having lsb_release
  installed, and with having lsb_release installed via the redhat-lsb
  package).

  Examples of triples on Linux after this change:

    # x86_64 Unbuntu 12.04 LTS:
    x86_64-ubuntu-linux-gnu

    # x86_64 Fedora 20 Desktop with redhat-lsb package installed
    x86_64-fedora-linux-gnu

    # x86_64 Fedora 20 Desktop without redhat-lsb-core installed
    # (i.e. no /bin/lsb_release available)
    # same as before the change
    x86_64--linux-gnu

  Note I intend to have Android respond with:

    {arch}-android-linux

  when I get to implementing Android lldb-gdbserver support.