deps: fix gypi sysroot settings on V8

On Node.js v8.x, gn will pass a sysroot parameter to clang to use a
downloaded sysroot files while running `make test-v8`. Recently,
chromium build tools switched to use Debian sid sysroot files instead of
Debian jessie. This patch updates our V8 GYP files to conform with those
changes.

Ref: https://github.com/nodejs/node/issues/21433

PR-URL: https://github.com/nodejs/node/pull/21494
Refs: https://github.com/nodejs/node/issues/21433


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Ujjwal Sharma <usharma1998@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>