FFI at CT (#10150)

* enable FFI at CT
* rename useFFI=>nimHasLibFFI; improve formatting rawExecute traceCode
* disable libffi on windows (works for win32, not yet win64)

compiler/options.nim

@@ -18,7 +18,7 @@ const
   hasTinyCBackend* = defined(tinyc)
   useEffectSystem* = true
   useWriteTracking* = false
-  hasFFI* = defined(useFFI)
+  hasFFI* = defined(nimHasLibFFI)
   copyrightYear* = "2018"
 
 type                          # please make sure we have under 32 options
@@ -128,6 +128,10 @@ type
     forLoopMacros,
     caseStmtMacros,
     codeReordering,
+    compiletimeFFI,
+      ## This requires building nim with `-d:nimHasLibFFI`
+      ## which itself requires `nimble install libffi`, see #10150
+      ## Note: this feature can't be localized with {.push.}
 
   SymbolFilesOption* = enum
     disabledSf, writeOnlySf, readOnlySf, v2Sf