[nanvix] E: Link libnvx_crt0.a into python.elf

[esaurez]

Summary

Stacks on top of nanvix/cpython#682. Adds libnvx_crt0.a -- the executable-startup archive introduced by the Nanvix nvx-crt0 crate split -- to python.elf's --whole-archive LIBS segment.

This makes python.elf forward-compatible with the planned Nanvix upstream change that removes _start / _do_start / c_trampoline from libposix.a: once libposix drops the duplicates, libnvx_crt0.a becomes the sole provider of those symbols. Today both archives ship them; with the existing -Wl,--allow-multiple-definition LDFLAG and the new ordering (libnvx_crt0 first), the linker picks the crt0 copy, so python.elf uses the same _start source across both the current and the future Nanvix state.

What changed in Makefile.nanvix

Section	Change
Variable definitions	New LIBNVX_CRT0 := .../lib/libnvx_crt0.a in both the docker and host-build branches.
Existence guard	Parse-time $(error ...) if $(LIBNVX_CRT0) is missing, gated on MAKECMDGOALS so clean and distclean still work against older sysroots.
LIBS line	Prepend $(LIBNVX_CRT0) to the --whole-archive group: -Wl,--whole-archive $(LIBNVX_CRT0) $(LIBPOSIX) $(LIBC) $(LIBM) $(LIBSTDCXX) $(LIBGCC) -Wl,--no-whole-archive ....
Comment block	Extended to document the ordering rationale and the future-state migration to a stripped-down libposix.a.

.nanvix/config.py::configure_env() (the dead-code helper kept in sync per the prior PR's docstring) is also updated to mirror the new LIBS line.

Ordering rationale (the subtle bit)

libnvx_crt0 is listed first inside --whole-archive so that under -Wl,--allow-multiple-definition, the linker picks the crt0-side copy of _start / _do_start / c_trampoline over the libposix-side copy. This is an intentional behaviour change:

• Today (libposix still ships startup symbols): _start comes from libnvx_crt0.a. Functionally identical to the old libposix-provided _start, but the source archive is different.
• Tomorrow (after the planned Nanvix upstream change removes startup from libposix): _start comes from libnvx_crt0.a -- now the sole provider.

Listing crt0 first means python.elf's startup behaviour does not silently change when the upstream cutover happens. The alternative (list crt0 last to preserve today's exact provider) would mean a subtle behaviour shift the day libposix changes.

Existence guard

The new guard fails the make-parse step with an actionable message if the sysroot lacks libnvx_crt0.a:

$(error libnvx_crt0.a not found at $(LIBNVX_CRT0). Update the Nanvix sysroot to one that ships libnvx_crt0.a ...)

Without the guard, a stale sysroot would lead to a confusing linker error ("file not found: libnvx_crt0.a") inside a long python configure log. With it, the user gets the message the moment they invoke make build.

The guard is gated:

ifdef CONFIG_NANVIX
ifneq ($(filter clean distclean,$(MAKECMDGOALS)),$(MAKECMDGOALS))
ifeq ($(wildcard $(LIBNVX_CRT0)),)
$(error ...)
endif
endif
endif

so it only fires when CPython is actually being built; make clean against an older sysroot still works.

Validation

• ./z build succeeds; python.elf grows by ~500 bytes (the small handful of crt0 objects added under --whole-archive; most of nvx-crt0's content was already in python.elf via libposix transitively).
• i686-nanvix-nm python.elf shows T _start and T _do_start resolved.
• numpy 1.26.4 import + np.arange + np.dot + reshape + broadcasting all work; the test harness prints NUMPY_TEST_OK on the Nanvix microvm.

Compatibility

• Requires a Nanvix sysroot that ships libnvx_crt0.a -- this is enforced by the existence guard.
• No change to CPython source code. Only the Nanvix-specific build harness (Makefile.nanvix + .nanvix/config.py).
• Multi-process / single-process / standalone modes unaffected (the linker changes are not mode-conditional).

Follow-ups

A separate Nanvix-side PR (next in the stack) will remove the startup symbols from libposix.a itself, after which libnvx_crt0.a becomes the only _start provider. The link line in this PR is already correct for that future state -- no further cpython change is needed.