Fix building TruffleRuby with TruffleRuby in $PATH

* Move --building-core-cexts next to other options in
  miniruby_for_building_cexts(), since it should be passed along those
  and needs --experimental-options.
* With TruffleRuby in $PATH, preprocess.rb would be called and fail due
  to --building-core-cexts being an experimental option.

Author: eregon

mx.truffleruby/mx_truffleruby.py

@@ -65,6 +65,7 @@ def miniruby_for_building_cexts(args):
     mx_binary = join(mx._mx_home, 'mx')
     options = [
         '--experimental-options',
+        '--building-core-cexts',
         '--home=' + root,
         '--launcher=' + mx_binary + ' -p ' + root + ' miniruby_for_building_cexts',
         '--disable-gems'

mx.truffleruby/suite.py

@@ -260,7 +260,6 @@
                 "sulong:SULONG_LEGACY", # truffle.h
             ],
             "buildEnv": {
-              "TRUFFLERUBYOPT": "--building-core-cexts",
               "NFI_HEADERS_DIR": "<path:truffle:TRUFFLE_NFI_NATIVE>/include",
               "SULONG_HEADERS_DIR": "<path:SULONG_HOME>/include",
               "SULONG_POLYGLOT_H": "<path:SULONG_HOME>/include/polyglot.h",