lib/std/Build.zig: add standard linkage option

[RossComputerGuy]

Adds a standardLinkageOption to the builder API. It's pretty handy if this was standardized.

[alexrp]

This seems useful enough to justify its existence to me.

However, it's going to take a lot of effort to make all existing projects adopt it. This is a problem we'll keep running into as we add more standard options. Maybe it's time to bundle them all into a single b.standardOptions() function that optimizes for the overwhelmingly common case where the user wants all of them?

[RossComputerGuy]

Yeah, I think a gradual shift to a standardOptions function would be best but we also have the standalone ones in case projects need more granular control. I also wonder if the zig compiler itself could make use of some of this logic to determine whether to default to static or dynamic linking if we move some stuff to std.Target.