This implements external memory using a Zircon VMO handle
that can be transferred between Fuchsia processes easily.

Bug: b/140419396
Change-Id: I81cecec35b218eb22f3318ddec7b790b89e5ce09
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/36168Tested-by: David Turner <digit@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>