platform-support.md: Update for consistency with Target Tier Policy

Split into five sections to match the tiers: "Tier 1 with Host Tools",
"Tier 1", "Tier 2 with Host Tools", "Tier 2", and "Tier 3". Explain each
tier briefly in prose, and link to the corresponding section of the
policy for full requirements.

Drop the `host` columns from the first four, since the different
sections distinguish that. (Keep the `host` column for "Tier 3", since
it's a single list and the `host` column just indicates if host tools
are expected to work.)

Targets with host tools always have full support for std, so drop the
`std` column from those.

Move the explanations of the `std` column next to the appropriate
tables, and drop the unknown/WIP case for tier 2 targets.

Use "target" terminology consistently throughout.

Sort each table by target name.

Author: joshtriplett