Ditch architectures that are failing to build 3.15+ on Alpine

[tianon]

• refs failure building Python 3.15.0a3 on Alpine python/cpython#143632

$ diff -u --color <(bashbrew cat python) <(bashbrew cat <(./generate-stackbrew-library.sh))
--- /dev/fd/63	2026-07-10 10:52:06.555577857 -0700
+++ /dev/fd/62	2026-07-10 10:52:06.555577857 -0700
@@ -24,12 +24,12 @@
 Directory: 3.15-rc/slim-bookworm
 
 Tags: 3.15.0b3-alpine3.24, 3.15-rc-alpine3.24, 3.15.0b3-alpine, 3.15-rc-alpine
-Architectures: amd64, arm32v6, arm32v7, arm64v8, i386, ppc64le, riscv64, s390x
+Architectures: amd64, arm64v8, riscv64, s390x
 GitCommit: 4f07059567fa61c80397126e8063dbbde011b794
 Directory: 3.15-rc/alpine3.24
 
 Tags: 3.15.0b3-alpine3.23, 3.15-rc-alpine3.23
-Architectures: amd64, arm32v6, arm32v7, arm64v8, i386, ppc64le, riscv64, s390x
+Architectures: amd64, arm64v8, riscv64, s390x
 GitCommit: 4f07059567fa61c80397126e8063dbbde011b794
 Directory: 3.15-rc/alpine3.23