If an address resolves to a user label in an isolated region, we
don't want to use it. However, we still want to try to match it
to a project/platform symbol.
For example, suppose the isolated code wants to reference address
$1C00, which is a memory-mapped I/O location in one area, but a
regular bunch of code in the other. We don't want it to map to
the regular code, but we do want it to resolve to our table of
platform I/O addresses.
We now handle this correctly. The regression test has been updated
to check this. The current implementation does a linear scan through
the symbol table, but I'm hoping this is not a common situation.
The reference manual has been updated to describe the new feature.
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