compiler,runtime: implement a portable conservative GC

src/runtime/gc_globals_conservative.go

@@ -0,0 +1,12 @@
+// +build gc.conservative
+// +build cortexm
+
+package runtime
+
+// markGlobals marks all globals, which are reachable by definition.
+//
+// This implementation marks all globals conservatively and assumes it can use
+// linker-defined symbols for the start and end of the .data section.
+func markGlobals() {
+	markRoots(globalsStart, globalsEnd)
+}