LLVM 7.0 already demands 3.4.3, and 3.6 improved Ninja support.

Bug: b/139189696
Change-Id: Ic94b6828e686646198d08bdd9bd886caa56a51df
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/31770Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>