Rework v8dbg constants support

Rework v8 debug constants support, to allow GDB, SystemTap and DTrace to use a common source for debug constants, by using a common header file which defines list macros which can be used to generate definitions of these constants in several different formats.

I've left in examples for generating a struct, an array + an enum, and the global definitions which are used currently.
Tested with both the struct and array + enum options, using GDB and SystemTap scripts.

Changes:

    - gen-postmortem-metadata.py: split off debug-support.h
      Convert debug constants to a list macro, and make the
      header file not depend on any other headers.
      This allows for easy preprocessing / parsing / inclusion.
      Define macros for easy generation of a struct or an array of
      debug constants, which can be accesses by any of GDB, SystemTap
      and DTrace scripts.
    - debug-support.cc: Add __v8dbg_init(), to be called when
      statically linking to libv8. Otherwise, the linker ignores
      debug-support.o, and no debug constants will be available in the
      final binary.
    - v8.cc: Add call to __v8dbg_init(). This should force the linker
      to include debug-support.o when statically linking libv8.
      Also, scripts should capture the debug constants while executing
      a probe on this function and not hardcode them, because the
      constants are essentially process-local (multiple processes,
      statically linked binaries linked with different libv8 versions
      could be active on the same system.)

Because this concerns V8 more than Node specifically, I would like to invite @bnoordhuis o have a look at this.
Any comments or suggestions are very welcome.