PR9968: Share global variable between embedded C and script

PR9968: Share global variable between embedded C and script

Embedded-C functions and blocks can now use the
STAP_GLOBAL_{GET,SET}_* macros to access script level global variables.

* NEWS: description
* doc/langref.text: description + added section for stable and
  unmodified-fnargs pragmas which was only in man/stap.1
* staptree.cxx: scan for pragmas in varuse_collecting_visitor
  visit_embeddedcode and visit_embedded_expr to allow proper locks
  to be emitted
* translate.cxx: c_unparser visit_embeddedcode and visit_embedded_expr
  generates the needed macros