[patch][pr10367] Add support for offsets in GlobalAlias

[Rafael Espíndola]

Thanks Reid and Tim.

An updated patch is attached. The main changes are
* Update documentation.
* Make it support 64 bit offsets.
* Handle offsets pointing out of objects and because of that, use a
signed offset.
* Updated globalopt to be aware of offsets.

The reason I decided to go with Reid's suggestion is that is valid on
ELF (and on some special cases on MachO) to have a relocation that
points outside of an object. Given that it seems natural to allow a
name (alias) for things like "the byte immediately after this string".

Cheers,
Rafael
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