stage1: implement --full-name

[ifreund]

This allows overriding the output name of the resulting binary, including the lib prefix for libraries and the extension.

This is necessary, for example, when creating a shared library for use as a plugin, in which case specific naming conventions must be followed.

Closes #2231
This does not yet expose this feature in the zig build system, that will be implemented in a follow up PR if this one is accepted

[daurnimator]

Should there be an error if both --name and --full-name are set?

[ifreund]

Should there be an error if both --name and --full-name are set?

Not opposed, but not sure if it's really needed. The current behavior of --full-name overriding --name makes sense IMO.

[ifreund]

It's worth noting that for my particular use case, overriding the prefix applied to the name alone would be sufficient when combined with #6315. Perhaps this option should keep the default extension and only override the lib prefix, as then produced binaries would still have the "proper" extension based on the target.

[ifreund]

Looks like this is already implemented in #6250, no point in implementing this in stage1