compiler: do not emit debug info for extern globals

This results in a link error in the following commit (undefined
reference to runtime.trackedGlobalsBitmap from .debug_info). Solution:
don't emit debug info for declared but not defined symbols.

compiler/symbol.go

@@ -82,7 +82,7 @@ func (c *Compiler) getGlobal(g *ssa.Global) llvm.Value {
 			}
 		}
 
-		if c.Debug() {
+		if c.Debug() && !info.extern {
 			// Add debug info.
 			// TODO: this should be done for every global in the program, not just
 			// the ones that are referenced from some code.