Merge pull request #2851 from stefantalpalaru/gogc

the Go GC (using the gccgo implementation)

doc/advopt.txt

@@ -58,7 +58,7 @@ Advanced options:
   --skipUserCfg             do not read the user's configuration file
   --skipParentCfg           do not read the parent dirs' configuration files
   --skipProjCfg             do not read the project's configuration file
-  --gc:refc|v2|markAndSweep|boehm|none
+  --gc:refc|v2|markAndSweep|boehm|go|none
                             select the GC to use; default is 'refc'
   --index:on|off            turn index file generation on|off
   --putenv:key=value        set an environment variable

doc/koch.txt

@@ -71,7 +71,7 @@ options:
   are limited to pure Nim code at compilation time.  Enabling
   this switch will allow macros to execute non-nim code at
   compilation time (eg. open a file and write to it).
---gc:refc|v2|markAndSweep|boehm|none
+--gc:refc|v2|markAndSweep|boehm|go|none
   Selects which garbage collection strategy to use for the compiler
   and generated code. See the `Nim's Garbage Collector <gc.html>`_
   documentation for more information.