all: rename garbage collectors

dumb -> leaking:
  make it more clear what this "GC" does: leak everything.
marksweep -> conservative:
  "marksweep" is too generic, use "conservative" to differentiate
  between future garbage collectors: precise marksweep / mark-compact /
  refcounting.

compiler/compiler.go

@@ -151,7 +151,7 @@ func (c *Compiler) TargetData() llvm.TargetData {
 func (c *Compiler) selectGC() string {
 	gc := c.GC
 	if gc == "" {
-		gc = "dumb"
+		gc = "leaking"
 	}
 	return gc
 }

main.go

@@ -537,7 +537,7 @@ func handleCompilerError(err error) {
 func main() {
 	outpath := flag.String("o", "", "output filename")
 	opt := flag.String("opt", "z", "optimization level: 0, 1, 2, s, z")
-	gc := flag.String("gc", "", "garbage collector to use (none, dumb, marksweep)")
+	gc := flag.String("gc", "", "garbage collector to use (none, leaking, conservative)")
 	panicStrategy := flag.String("panic", "print", "panic strategy (abort, trap)")
 	printIR := flag.Bool("printir", false, "print LLVM IR")
 	dumpSSA := flag.Bool("dumpssa", false, "dump internal Go SSA")

src/runtime/gc_conservative.go

@@ -1,4 +1,4 @@
-// +build gc.marksweep
+// +build gc.conservative
 
 package runtime
 

src/runtime/gc_leaking.go

@@ -1,4 +1,4 @@
-// +build gc.dumb
+// +build gc.leaking
 
 package runtime
 

targets/cortex-m.json

@@ -3,7 +3,7 @@
 	"goos": "linux",
 	"goarch": "arm",
 	"compiler": "clang",
-	"gc": "marksweep",
+	"gc": "conservative",
 	"linker": "ld.lld",
 	"rtlib": "compiler-rt",
 	"cflags": [