GeezerVison

Fixes #154200

test_tagged_suffix computes its expected extension suffix with a vendored
"Port of distutils.util.get_platform()" that sniffs sys.version.lower() for 'amd64'.

Python/getversion.c truncates the compiler segment of sys.version at 80 characters
("%.80s"). On clang-cl builds the banner is
"[Clang " __clang_version__ "] 64 bit (AMD64) with MSC v.<n> CRT]", and official LLVM
Windows binaries have an 86-character __clang_version__, so the (AMD64) marker is always
cut off. The helper falls through to return sys.platform -> 'win32', and the test expects
.cp316t-win32.pyd while the interpreter correctly reports .cp316t-win_amd64.pyd:

AssertionError: '.cp316t-win32.pyd' not found in ['.cp316t-win_amd64.pyd', '.pyd']

sysconfig.get_platform() no longer has this problem: since gh-145410 it does not parse
sys.version on Windows but delegates to the _sysconfig C accelerator, which derives the
platform from compile-time architecture macros and is compiler-agnostic. So this uses it and
deletes the vendored copy, which that call site was the only consumer of.

This is not circular: SYSCONFIG_PLATFORM in Modules/_sysconfig.c and PYD_PLATFORM_TAG in
PC/pyconfig.h are independent definitions in separate files, so the assertion still
cross-checks two separate derivations.

It also drops the VSCMD_ARG_TGT_ARCH branch, which trusted the ambient Visual Studio
developer-prompt target architecture over the interpreter actually executing the test.
That was backwards — EXTENSION_SUFFIXES is compiled into the running binary — and it is
why the failure only reproduced outside a VS developer prompt.

Testing

On a local clang-cl free-threaded build, python -m unittest test.test_importlib.test_windows:

• before: Ran 11 tests ... FAILED (failures=2) (the Frozen_ and Source_ variants)
• after: Ran 11 tests ... OK

Also OK with VSCMD_ARG_TGT_ARCH=x64, and with VSCMD_ARG_TGT_ARCH=x86 deliberately
mismatched against this amd64 interpreter — the old code would have computed 'win32' and
failed there. Full python -m test test_importlib: run=1229 skipped=23, SUCCESS.

Note the re.sub() is still required: sysconfig.get_platform() returns win-amd64 with a
hyphen while PYD_PLATFORM_TAG uses win_amd64. os and re both remain in use elsewhere
in the module, so no imports become unused.

NEWS

This is a test-only change, which the devguide lists as not requiring a Misc/NEWS.d entry,
so I have not added a blurb — could a maintainer apply skip news? Happy to add one instead
if you would prefer, on the grounds that it unblocks a whole build configuration.

• Issue: test_importlib.test_windows: vendored get_platform() misdetects clang-cl builds as win32, failing test_tagged_suffix #154200