tools: install gdbinit from v8 to $PREFIX/share

gdbinit provided by V8 can be very useful for low-level debugging of
crashes in node and in binary addons. Most useful commands at 'jst'
for JS stack traces and 'job' for printing a heap object.

This patch installs the file at $PREFIX/share/doc/node/gdbinit.

Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
PR-URL: https://github.com/nodejs/io.js/pull/2123