cmd/link: move symtab out of mapped memory

<pre>Nothing in package runtime looks at the Go 'symtab' symbol anymore. Move it into
unmapped memory, so that it is still in the file for debuggers but not mapped while the
program runs.

Low priority, since the size of the file will not be changing, and since the data is
never paged in, the total memory cost is under two pages of memory partially used by
fragmentation, so less than 8 kB.</pre>
