add mm to compilesettings; deprecate gc (#19394)

compiler/vmops.nim

@@ -135,6 +135,7 @@ when defined(nimHasInvariant):
     of backend: result = $conf.backend
     of libPath: result = conf.libpath.string
     of gc: result = $conf.selectedGC
+    of mm: result = $conf.selectedGC
 
   proc querySettingSeqImpl(conf: ConfigRef, switch: BiggestInt): seq[string] =
     template copySeq(field: untyped): untyped =

lib/std/compilesettings.nim

@@ -32,7 +32,8 @@ type
     backend           ## the backend (eg: c|cpp|objc|js); both `nim doc --backend:js`
                       ## and `nim js` would imply backend=js
     libPath           ## the absolute path to the stdlib library, i.e. nim's `--lib`, since 1.5.1
-    gc                ## gc selected
+    gc {.deprecated.} ## gc selected
+    mm                ## memory management selected
 
   MultipleValueSetting* {.pure.} = enum ## \
                       ## settings resulting in a seq of string values

tests/vm/tcompilesetting.nim

@@ -1,5 +1,5 @@
 discard """
-cmd: "nim c --nimcache:build/myNimCache --nimblePath:myNimblePath $file"
+cmd: "nim c --nimcache:build/myNimCache --nimblePath:myNimblePath --gc:arc $file"
 joinable: false
 """
 
@@ -12,6 +12,7 @@ template main =
   doAssert "myNimblePath" in nimblePaths.querySettingSeq[0]
   doAssert querySetting(backend) == "c"
   doAssert fileExists(libPath.querySetting / "system.nim")
+  doAssert querySetting(mm) == "arc"
 
 static: main()
 main()