enable testing -d:nimHasLibFFI mode (#13091)

compiler/condsyms.nim

@@ -103,3 +103,9 @@ proc initDefines*(symbols: StringTableRef) =
   defineSymbol("nimNewShiftOps")
   defineSymbol("nimHasCursor")
   defineSymbol("nimHasExceptionsQuery")
+
+  when defined(nimHasLibFFI):
+    # Renaming as we can't conflate input vs output define flags; e.g. this
+    # will report the right thing regardless of whether user adds
+    # `-d:nimHasLibFFI` in his user config.
+    defineSymbol("nimHasLibFFIEnabled")

compiler/evalffi.nim

@@ -17,6 +17,8 @@ when defined(windows):
   const libcDll = "msvcrt.dll"
 elif defined(linux):
   const libcDll = "libc.so(.6|.5|)"
+elif defined(bsd):
+  const libcDll = "/lib/libc.so.7"
 elif defined(osx):
   const libcDll = "/usr/lib/libSystem.dylib"
 else: