builder: refactor job runner and use a shared semaphore across build jobs

Switching to a shared semaphore allows multi-build operations (compiler tests, package tests, etc.) to use the expected degree of parallelism efficiently.

While refactoring the job runner, the time complexity was also reduced from O(n^2) to O(n+m) (where n is the number of jobs, and m is the number of dependencies).

compileopts/options.go

@@ -32,7 +32,7 @@ type Options struct {
 	DumpSSA         bool
 	VerifyIR        bool
 	PrintCommands   func(cmd string, args ...string)
-	Parallelism     int // -p flag
+	Semaphore       chan struct{} // -p flag controls cap
 	Debug           bool
 	PrintSizes      string
 	PrintAllocs     *regexp.Regexp // regexp string