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

lib/system/sysstr.nim

@@ -122,6 +122,8 @@ proc copyStringRC1(src: NimString): NimString {.compilerRtl.} =
       result = cast[NimString](newObjRC1(addr(strDesc), sizeof(TGenericSeq) +
                                s+1))
       result.reserved = s
+      when defined(gogc):
+        result.elemSize = 1
     else:
       result = rawNewStringNoInit(src.len)
     result.len = src.len