gdbhooks: Handle references to vec* in VecPrinter

gdbhooks: Handle references to vec* in VecPrinter

vec.h has this method:

  template<typename T, typename A>
  inline T *
  vec_safe_push (vec<T, A, vl_embed> *&v, const T &obj CXX_MEM_STAT_INFO)

where v is a reference to a pointer to vec.  This matches the regex for
VecPrinter, so gdbhooks.py attempts to print it but chokes on the reference.
I see the following:

  #1  0x0000000002b84b7b in vec_safe_push<edge_def*, va_gc> (v=Traceback (most
  recent call last):
    File "$SRC/gcc/gcc/gdbhooks.py", line 486, in to_string
      return '0x%x' % intptr(self.gdbval)
    File "$SRC/gcc/gcc/gdbhooks.py", line 168, in intptr
      return long(gdbval) if sys.version_info.major == 2 else int(gdbval)
  gdb.error: Cannot convert value to long.

This patch makes VecPrinter handle such references by stripping them
(dereferencing) at the top of the relevant functions.

gcc/ChangeLog:

	* gdbhooks.py (strip_ref): New. Use it ...
	(VecPrinter.to_string): ... here,
	(VecPrinter.children): ... and here.