Exclude "s390x" for bullseye+ (due to libffi6 vs libffi7)

[tianon]

pypy3: error while loading shared libraries: libffi.so.6: cannot open shared object file: No such file or directory

Diff:

$ diff -u <(bashbrew cat pypy) <(bashbrew cat <(./generate-stackbrew-library.sh))
--- /dev/fd/63	2021-08-20 11:29:32.119968516 -0700
+++ /dev/fd/62	2021-08-20 11:29:32.119968516 -0700
@@ -3,12 +3,12 @@
 
 Tags: 3.7-7.3.5-bullseye, 3.7-7.3-bullseye, 3.7-7-bullseye, 3.7-bullseye, 3-7.3.5-bullseye, 3-7.3-bullseye, 3-7-bullseye, 3-bullseye, bullseye
 SharedTags: 3.7-7.3.5, 3.7-7.3, 3.7-7, 3.7, 3-7.3.5, 3-7.3, 3-7, 3, latest
-Architectures: amd64, arm64v8, i386, s390x
+Architectures: amd64, arm64v8, i386
 GitCommit: 2c7223f5fc2fba18bc8e5cb14896668a5c330ae1
 Directory: 3.7/bullseye
 
 Tags: 3.7-7.3.5-slim, 3.7-7.3-slim, 3.7-7-slim, 3.7-slim, 3-7.3.5-slim, 3-7.3-slim, 3-7-slim, 3-slim, slim, 3.7-7.3.5-slim-bullseye, 3.7-7.3-slim-bullseye, 3.7-7-slim-bullseye, 3.7-slim-bullseye, 3-7.3.5-slim-bullseye, 3-7.3-slim-bullseye, 3-7-slim-bullseye, 3-slim-bullseye, slim-bullseye
-Architectures: amd64, arm64v8, i386, s390x
+Architectures: amd64, arm64v8, i386
 GitCommit: 2c7223f5fc2fba18bc8e5cb14896668a5c330ae1
 Directory: 3.7/slim-bullseye