checksharedmods fails on non-glibc cross builds (host interpreter gets target LD_LIBRARY_PATH) #160924

Bug report

Cross-compiling 3.15 for a musl target (glibc build machine) fails in make at the checksharedmods target:

nativepython3: error while loading shared libraries: libc.so: cannot open shared object file: No such file or directory
make: *** [Makefile:1801: checksharedmods] Error 127

The rule runs the build python under RUNSHARED:

@$(RUNSHARED) $(PYTHON_FOR_BUILD) $(srcdir)/Tools/build/check_extension_modules.py

In a cross build PYTHON_FOR_BUILD is the host interpreter (from --with-build-python) but RUNSHARED points LD_LIBRARY_PATH at the target's build dir, so the host interpreter gets the target libraries. On glibc->glibc nobody notices, but with a musl target the glibc host binary can't load the musl libc.so and dies before Python even starts.

This started with #141926 / GH-141958, which dropped the old

if test "$cross_compiling" = yes; then
  RUNSHARED=
fi

That PR was about getting PGO to work under emulation, but checksharedmods runs in the normal build_all path even without PGO, so now every non-emulated cross build to a different libc breaks here.

As far as I can tell check_extension_modules.py doesn't actually need the target libs on LD_LIBRARY_PATH when cross-compiling — it already takes the cross path (via _PYTHON_HOST_PLATFORM) and only checks that the module files exist, it doesn't import them.

Putting the cross_compiling RUNSHARED reset back makes the build work again. Would it make sense to restore it, or to only use RUNSHARED for the build-python when a HOSTRUNNER is set?

CPython versions tested on:

3.15

Operating systems tested on:

Linux