src: don't use NewExternal() with unaligned strings

V8 3.20.9 enforces that external pointers are aligned on a two-byte
boundary.

We cannot portably guarantee that for the source code strings that
tools/js2c.py generates so simply stop using String::NewExternal()
altogether (and by extension String::ExternalAsciiStringResource).

Fixes the following run-time assert:

  FATAL ERROR: v8::String::NewExternal() Pointer is not aligned