gogc: GCC-8.2.0 compatibility and other improvements (#9211)

- Go's write barriers are now plugged-in in all the relevant points
- "gcGo" is correctly classified by usesWriteBarrier()
- some gogc structures and functions now use golib wrappers to keep GCC
  version-specific conditions out of the compiler/stdlib code
- we no longer allow mixing the C malloc with Go's
- fix a problem with string copying

compiler/options.nim

@@ -104,8 +104,10 @@ type
     cmdJsonScript             # compile a .json build file
   TStringSeq* = seq[string]
   TGCMode* = enum             # the selected GC
-    gcNone, gcBoehm, gcGo, gcRegions, gcMarkAndSweep, gcDestructors,
-    gcRefc, gcV2
+    gcNone, gcBoehm, gcRegions, gcMarkAndSweep, gcDestructors,
+    gcRefc, gcV2, gcGo
+    # gcRefc and the GCs that follow it use a write barrier,
+    # as far as usesWriteBarrier() is concerned
 
   IdeCmd* = enum
     ideNone, ideSug, ideCon, ideDef, ideUse, ideDus, ideChk, ideMod,