Support for ppc64le

### Is your feature request related to a problem? yes

When we try to build the project on ppc64le platform, we get errors.
### Describe the solution you'd like

We get error saying "platform not supported". "architecture not supported" etc.. I think we need to modify compiler/check-artifact.sh to include support for ppc64le also. Similarly one of the test files testing/src/main/java/io/grpc/internal/testing/TestUtils.java also doing arch specific checks. I think that also needs modification
### Describe alternatives you've considered

None
### Additional context

While trying to build and test the project IBM's power linux(ppc64le) I got into these issues

Start by reproducing the ppc64le build and test failures described in the issue. Inspect compiler/check-artifact.sh and the architecture-specific checks in testing/src/main/java/io/grpc/internal/testing/TestUtils.java. Done means the project builds and tests successfully on ppc64le without platform or architecture-not-supported errors.

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