Update dockerfiles for emsdk#4771

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mattsoulanille wants to merge 15 commits intotensorflow:masterfrom

mattsoulanille:update_dockerfiles

Commits on Feb 25, 2021

Compile a simple hello world example using emsdk's bazel toolchain
mattsoulanille
committed
xnnpack hello world not working with emsdk toolchain
mattsoulanille
committed

Commits on Feb 26, 2021

Remove unused variables
mattsoulanille
committed