fix some issues with --backend (#14363)

* fix some issues with --backend
* fix https://github.com/timotheecour/Nim/issues/175; improve upon #14306

compiler/options.nim

@@ -223,7 +223,7 @@ type
                           ## fields marked with '*' are subject to
                           ## the incremental compilation mechanisms
                           ## (+) means "part of the dependency"
-    backend*: TBackend
+    backend*: TBackend # set via `nim x` or `nim --backend:x`
     target*: Target       # (+)
     linesCompiled*: int   # all lines that have been compiled
     options*: TOptions    # (+)
@@ -419,7 +419,7 @@ proc newConfigRef*(): ConfigRef =
     cIncludes: @[],   # directories to search for included files
     cLibs: @[],       # directories to search for lib files
     cLinkedLibs: @[],  # libraries to link
-    backend: backendC,
+    backend: backendInvalid,
     externalToLink: @[],
     linkOptionsCmd: "",
     compileOptionsCmd: @[],