siggen: Fix hashequiv performance issues

We're seeing huge slowdowns on large builds on the autobuilder. A qemux86 world
build was running really slowly, a primary feature was lots of rehashing going on
due to an sstate change which caused a rebuild when all output should be identical.

This was traced to the hundreds of thousands of calls to get_unihash() from
get_taskash(). If we simplify the unitaskhashes data structure, we can bypass
the function call and access the data directly. In local profile charts,
this significanly sped up process_possible_migrations(), both on profiles
and visually.

Whilst this change doesn't aid readability, it does solve an otherwise
huge performance issue.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

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