All is used in your sentence as a pronoun, and all as a pronoun can be either singular or plural, you can also refer to this article: https://www.englishgrammar101.com/module-2/pronouns/lesson-11/indefinite-pronoun-agreement
As it is mentioned in the article:
With pronouns that can be either singular or plural, you need to look closely at the prepositional phrase of the + noun in order to determine whether the meaning is singular or plural.
In your sentence however this is not made explicit, so it is pretty much down to what interpretation is given or common usage.
All I got for Christmas was stocks and a computer.
is acceptable and might even be preferred as many would read 'all' as 1 collection of items which comprises stocks and a computer.
All I got for Christmas were stocks and a computer.
is also acceptable if we read 'all' as multiple items, e.g. if 'all' would stand for '(all of) the presents that I received for Christmas [...]'.