testing/mdcat: disable on ppc64le due to build failures

Log below:

error: failed to run custom build command for `ring v0.16.19`

Caused by:
  process didn't exit successfully: `/home/buildozer/aports/testing/mdcat/src/mdcat-mdcat-0.22.2/target/release/build/ring-c5c457031cdaa898/build-script-build` (exit code: 101)
  --- stderr
  thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', /home/buildozer/.cargo/registry/src/github.com-1ecc6299db9ec823/ring-0.16.19/build.rs:375:10
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
warning: build failed, waiting for other jobs to finish...
error: build failed

testing/mdcat/APKBUILD

@@ -5,7 +5,9 @@ pkgver=0.22.2
 pkgrel=0
 pkgdesc="Cat for markdown"
 url="https://github.com/lunaryorn/mdcat"
-arch="all !s390x !mips !mips64" # Limited by cargo
+# s390x, mips, mips64 limited by cargo
+# ppc64le fails to build 'ring v0.16.19'
+arch="all !s390x !mips !mips64 !ppc64le" 
 license="Apache-2.0"
 makedepends="cargo openssl-dev"
 checkdepends="py3-ansi2html"