Support compile: boolean | "${platform}-${arch}" in Bun.build

[Jarred-Sumner]

What does this PR do?

This lets you programmatically invoke bun build --compile.

await Bun.build({
  entryPoints: ["./index.ts"],
  compile: true,
});

In this first version, only a single target can be specified per build. This limitation exists mostly because some code needs to be refactored to allow multiple concurrent cross compilations effectively (since we use custom --define for process.os and process.arch, that means we cannot reuse the same ASTs across builds).

Fixes: #11895

TODO before merging:

• Write a couple tests
• Figure out what to do about error messages / logs. it's not compiling right now.

How did you verify your code works?

[robobun]

❌ @Jarred-Sumner, your commit 3123df3 has some failures in #2538

[kravetsone]

Related - #11895