Prefix global functions in the generated code.

Prefix global functions in the generated code.

Add '__global_' prefix to global functions in the generated code.
Treat selected internal synthetic functions as private.
Fix using a private function in user script and test-cover it.

* elaborate.cxx: Correctly match global function names to respective
  functiondecls. Match synthetic private functions coming from
  tapset-procfs.cxx (*_procfs_value_*), and tapsets.cxx
  (_sdt_arg_get_addr_) to respective functiondecls.
* main.cxx: Update the --dump-functions feature to correctly deal with
  new internal function names.
* parse.cxx: Add the "__global_" prefix to global functions in the
  generated code.
* staptree.cxx: Update functiondecl::printsig() so that it outputs
  unmangled function name.
* tapset-procfs.cxx: Treat internal synthetic functions
  '*_procfs_value_get*', '*_procfs_value_set*', '*_procfs_value_append*'
  as private.
* tapsets.cxx: Treat internal synthetic functions '*_sdt_arg_get_addr_*'
  as private.
* optim_arridx.exp: Adapt the testcase to new naming.
* private.exp: Add subtest examining use of a private function in
  user script.