arm64: Remove pmap_san_bootstrap() and call kasan_init_early() directly

[markj]

Summary

pmap_san_bootstrap() doesn't really do much, and it was hard-coding the
the bootstrap stack size defined in locore.S. Moreover, the name is a
bit confusing given the existence of pmap_bootstrap_san(). Just remove
it and call kasan_init_early() directly like we do on amd64.

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