builder,interp: fix -ldflags -X not overriding variables with default values

[abgoyal]

Currently, -ldflags "-X pkg.Var=value" fails to override string variables that have a default value in source code:

var Version = "dev"  // -X override is ignored, stays "dev"
var Version string   // -X override works correctly

In standard Go, -X works in both cases. This patch brings TinyGo's behavior in line with Go.

The root cause is the timing of when -X values are applied relative to the interp pass. The previous approach:

1. Erase global, make it undefined
2. Run interp (executes Version = "dev" from init code)
3. Link in -X value via makeGlobalsModule

By step 2, interp had already stored the default value, and the linking step couldn't reliably override it.

This patch changes the approach:

1. Set -X value as initializer before interp runs
2. Run interp, but skip stores to -X globals
3. Global retains the -X value

This also means dependent compile-time computations work correctly:

var Version = "dev"
var VersionLen = len(Version)  // now evaluates with -X value

With this fix, makeGlobalsModule becomes redundant - we no longer need a separate module to work around type renaming issues since we set the value directly on the global using its existing type during per-package compilation.

I've removed makeGlobalsModule as part of this change. If there are edge cases I haven't considered where keeping it would be safer, happy to restore it as a fallback.