deps: update v8 to 3.15.11.7
v8: remove optimization switches

Remove compiler switches from $(TOPLEVEL)/deps/v8/build/common.gypi,
we set them globally in $(TOPLEVEL)/common.gypi.

Commit 7b4d95a introduced the switches again, resulting in V8 getting
built without any optimizations.

This commit is essentially a rehash of commit 4b8629d.