- Changed longjmp.s to export symbol __longjmp instead of _longjmp so th...

...at in header a macro longjmp can be used to hide function prototype and convert calls by value to calls by reference according to assembly implementation

- Changed header setjmp.h: Changed prototypes of _setjmp and _longjmp to receive jmp_buf by reference in agreement with assembly implementation
  - New macro definition #define longjmp(x,y) _longjmp(&(x),(y)) to imitate passing jmp_buf by value although in reality jmp_buf is passed by reference
  - Changed macro definition setjmp(x) to #define setjmp(x) _setjmp(&(x)) for the same reason
  - The new macro definitions also fix another issue when including setjmp.h more than once: the second include is not preprocessed to #include <_setjmp.h> but
    remaines #include <setjmp.h>