Added distribution info to ArchSpec and qHostInfo message.

ArchSpec now contains an optional distribution_id, with getters and
setters. Host::GetArchitecture () sets it on non-Apple platforms using
Host::GetDistributionId (). The distribution_id is ignored during
ArchSpec comparisons.

The gdb remote qHostInfo message transmits it, if set, via the
distribution_id={id-value} key/value pair. Updated gdb remote docs to
reflect this change.

As before, GetDistributionId () returns nothing on non-Linux platforms
at this time. On Linux, it is returned only if the lsb_platform
command is installed (in /bin or /usr/bin), and only if the
distributor id key is returned by 'lsb_platform -i'. This id is
lowercased, and whitespace is replaced with underscores.