Enable the mmap stdlib module on Nanvix

[ada-x64]

Closes #301.

Drops mmap from PY_STDLIB_MOD_SET_NA for the Nanvix arm and adds the one autoconf check needed to make the build link cleanly. Two commits: switch flip, then the HAVE_MSYNC guard that lets the link succeed.

What this enables

The mmap module imports. Most of it works against the existing libposix mmap binding (which today supports MAP_PRIVATE | MAP_ANONYMOUS with fd == -1):

• import mmap; all constants (PAGESIZE, MAP_*, PROT_*, ACCESS_*, MADV_*).
• mmap.mmap(-1, size) — anonymous private mappings. CPython's POSIX constructor (mmapmodule.c:1318) treats fd == -1 as "give me anonymous memory" and adds MAP_ANONYMOUS itself, so this hits libposix's supported path end-to-end.
• All buffer operations on the resulting object: read, write, seek, tell, find/rfind, indexing, slicing, len(), iteration, move, read_byte, write_byte, close, size.
• Use as a buffer-protocol object in any API that accepts one (struct, pickle, hashlib.update, …).
• .resize() for anonymous mappings (shrink works; grow uses the upstream unmap+remap fallback since mremap is absent).

What this doesn't enable, and why

Each of these is a missing piece in nanvix/nanvix, not in this PR; tracking issues filed:

• mmap.mmap(fd, …) with fd >= 0 → OSError(EINVAL). libposix's binding rejects every non-anonymous form because the kernel vmap kcall takes no fd/offset. [libs/kernel] Implement file-backed mmap() (non-anonymous mappings) nanvix#2211 (file-backed mmap).
• MAP_SHARED and MAP_FIXED flags → EINVAL. Same root cause. [libs/kernel] Implement file-backed mmap() (non-anonymous mappings) nanvix#2211.
• mmap.flush() → ValueError("flush not supported on this system"). msync is not in libposix; this PR's second commit gates the one call site (mmapmodule.c:683) on HAVE_MSYNC and falls through to the existing upstream #else path that already raises this exact ValueError on every other Unix without msync. [libs] Implement msync(2) nanvix#2212.
• mmap.madvise() → method absent. madvise not in libposix; upstream already gates the method on HAVE_MADVISE so it silently disappears. [libs] Implement madvise(2) nanvix#2213.
• mmap.resize() grow path on (eventual) file-backed mappings would lose contents. mremap not in libposix; upstream fallback only works for anonymous mappings. [libs] Implement mremap(2) nanvix#2214.
• mlock/munlock family — not surfaced via mmap at all (lives in resource); filed for completeness as [libs] Implement mlock/munlock/mlockall/munlockall nanvix#2215.

Why this shape (the second commit)

Modules/mmapmodule.c:683 calls msync() unconditionally on the UNIX branch with no upstream HAVE_MSYNC guard — even though the surrounding madvise/mremap code is all HAVE_*-gated and mmap/mremap are already in the AC_CHECK_FUNCS list right next door. msync is the anomaly.

Without the guard, the build doesn't link (verified: clean ./z distclean && ./z clean && ./z --mode=standalone setup && ./z build fails with Modules/mmapmodule.c:683: undefined reference to msync; nm on every .a in the freshly-set-up sysroot confirms zero msync definitions). With the guard, link succeeds and only mmap.flush() is affected — falling through to the same ValueError users on any other msync-less Unix already see.

Behaviour on every existing in-tree target is bit-identical: their autoconf detects msync, HAVE_MSYNC is defined, the original branch runs.

Per port convention, configure.ac and the generated configure are hand-edited in parallel rather than regenerated with autoreconf.

Verification

All three modes, full cycle (./z distclean && ./z clean && ./z --mode=$MODE setup && ./z build && ./z test): standalone (116 regrtest modules), single-process (162), multi-process (162) all green. mmap moves from the n/a list into the built-in set (38 → 37 n/a); no other module's status changes.

Out of scope

• Adding test_mmap to NANVIX_TEST_LIST. The module builds and the anonymous path works; wiring up the test will need an is_nanvix → SkipTest guard for the file-backed and MAP_SHARED cases, analogous to upstream's is_emscripten. Separate PR by convention.
• _posixshmem (Enable _posixshmem stdlib module (POSIX shared memory) #306) and other related modules.

[Copilot]

Pull request overview

Enables the mmap stdlib extension module for the Nanvix port by removing it from Nanvix’s “not available” module set, and fixes the resulting link failure by autoconf-detecting msync() and compiling mmap.flush()’s Unix implementation only when msync is available.

Changes:

• Remove mmap from PY_STDLIB_MOD_SET_NA for Nanvix so it can be built when headers/requirements are present.
• Add msync to AC_CHECK_FUNCS and introduce HAVE_MSYNC to the config header template.
• Gate the Unix mmap.flush() implementation behind HAVE_MSYNC, falling back to the existing “flush not supported” behavior when absent.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
pyconfig.h.in	Adds HAVE_MSYNC placeholder so autoconf can define it when detected.
configure.ac	Checks for msync() and stops forcing mmap to n/a on Nanvix.
configure	Mirrors configure.ac updates: emits HAVE_MSYNC and removes Nanvix mmap n/a assignment.
Modules/mmapmodule.c	Compiles the Unix msync()-based flush() path only when HAVE_MSYNC is defined.

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[ppenna]

Why is this required?