build: use host gn and ninja on non-mainstream architectures (#10248)
build: use host gn and ninja on non-mainstream architectures (#10248)

This simplifies build process on such architectures.
According to

    https://github.com/envoyproxy/envoy/pull/9527#discussion_r366623844

this approach is preferred to bundling binaries for each
such architecture.

Signed-off-by: Ilya Leoshkevich <425ffc1422dc4f32528bd9fd5af355fdb5c96192@linux.ibm.com>
istio/envoy,lizan/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy-wasm,lyft/envoy,lyft/envoy,istio/envoy,envoyproxy/envoy,lyft/envoy,istio/envoy,lizan/envoy,envoyproxy/envoy,lizan/envoy,envoyproxy/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy,lizan/envoy,envoyproxy/envoy,lizan/envoy,envoyproxy/envoy,lyft/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy-wasm,envoyproxy/envoy-wasm,envoyproxy/envoy,istio/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy,istio/envoy,istio/envoy,envoyproxy/envoy,lizan/envoy,envoyproxy/envoy,lyft/envoy,lyft/envoy,envoyproxy/envoy,envoyproxy/envoy,istio/envoy