cpython: have powerpc64le use "ppc64le" to follow PEP600

The PEP600 standard gives Python's naming scheme for various
architectures; it follows the convention which was in use by Fedora in
2014. According to PEP600, the architecture name for Power PC is
`ppc64le`, not `powerpc64le`. This is also how python3 declares its
"supported wheels" under Debian on PowerPC, as checked with `pip debug
--verbose`

$ pip debug --verbose | grep powerpc
$ pip debug --verbose | grep ppc | head
cp39-cp39-manylinux_2_31_ppc64le
cp39-cp39-manylinux_2_30_ppc64le
cp39-cp39-manylinux_2_29_ppc64le
cp39-cp39-manylinux_2_28_ppc64le
cp39-cp39-manylinux_2_27_ppc64le
cp39-cp39-manylinux_2_26_ppc64le
cp39-cp39-manylinux_2_25_ppc64le
cp39-cp39-manylinux_2_24_ppc64le
cp39-cp39-manylinux_2_23_ppc64le

Let's adjust the `pythonHostPlatform` expression in
cpython/default.nix to pass the architecture using the naming scheme
Python expects.

Verified on a Raptor Computing Systems Talos II. Without this commit,
PyQt5 fails to build, failing with "unsupported wheel". With this
commit, it builds successfully.

pkgs/development/interpreters/python/cpython/default.nix

···
     # The configure script uses "arm" as the CPU name for all 32-bit ARM
     # variants when cross-compiling, but native builds include the version
     # suffix, so we do the same.
-    pythonHostPlatform = "${parsed.kernel.name}-${parsed.cpu.name}";
+    pythonHostPlatform = let
+      cpu = {
+        # According to PEP600, Python's name for the Power PC
+        # architecture is "ppc", not "powerpc".  Without the Rosetta
+        # Stone below, the PEP600 requirement that "${ARCH} matches
+        # the return value from distutils.util.get_platform()" fails.
+        # https://peps.python.org/pep-0600/
+        powerpc = "ppc";
+        powerpcle = "ppcle";
+        powerpc64 = "ppc64";
+        powerpc64le = "ppc64le";
+      }.${parsed.cpu.name} or parsed.cpu.name;
+    in "${parsed.kernel.name}-${cpu}";
 
     # https://github.com/python/cpython/blob/e488e300f5c01289c10906c2e53a8e43d6de32d8/configure.ac#L724
     multiarchCpu =