feat(configure): enable posix_dir for *-nanvix*

[esaurez]

Summary

Enables posix_dir=posix for *-nanvix* in newlib/configure.host so the existing newlib/libc/posix/ translation units get compiled into libc.a, together with the matching set of newlib opt-out capability macros that keep the wider compile surface clean.

This is the change that unblocks the original rpmatch.c undefined-reference chain (regcomp, regexec, regerror, regfree) we hit while statically linking cpython on Nanvix, and adds the rest of the POSIX surface that nanvix-side code (cpython, future .so extensions) expects to find in libc.

What gets enabled

After this change, libc.a for *-nanvix* now defines (previously missing):

• Regex family: regcomp, regerror, regexec, regfree
• Pattern matching: fnmatch
• Misc POSIX: sleep, isatty / _isatty, creat, alphasort

Verified by rebuilding newlib under ghcr.io/nanvix/toolchain-gcc and running i686-nanvix-nm /opt/nanvix/i686-nanvix/lib/libc.a.

Why the opt-out cflags

Setting posix_dir=posix would otherwise drag in six translation units that cannot work on Nanvix today. Each one is silenced with a standard newlib capability macro — no new macro introduced. The shape mirrors the RTEMS section at configure.host:623 which uses the same idiom for a similar capability gap.

Flag	File silenced	Reason
-D_NO_POSIX_SPAWN	posix_spawn.c	needs fork / execve; also references sigprocmask unconditionally (same root cause as the prerequisite _NO_SIGPROCMASK change on this stack's base branch)
-D_NO_POPEN	popen.c	needs fork + pipe + execve
-D_NO_WORDEXP	wordexp.c	shells out to /bin/sh
-D_NO_GLOB	glob.c	glob / globfree reference getpwnam, issetugid, and getlogin, none of which nanvix's libposix provides today
-D_NO_EXECVE	execl.c, execle.c, execv.c, execve.c, execlp.c, execvp.c	all reference the SYS_execve syscall wrapper (_execve) which nanvix's libposix does not provide today
-DHAVE_OPENDIR	opendir.c, readdir.c, closedir.c, scandir.c, seekdir.c, telldir.c, rewinddir.c, dirfd.c, ftw.c, nftw.c	nanvix already provides its own opendir / readdir / closedir / ... in libposix, using a different struct dirent layout (no d_reclen). Newlib's BSD-style versions assume d_reclen and won't even compile against the nanvix <sys/dirent.h>. HAVE_OPENDIR makes the newlib copies empty so the symbols resolve from libposix at link time.

The dirent split is intentional on the nanvix side — see newlib/libc/sys/nanvix/include/sys/dirent.h which defines struct dirent (POSIX-API surface: d_ino + d_name) separately from struct posix_dent (on-disk form with d_reclen + d_type). The conversion happens inside libposix's own readdir.

How the _NO_* / HAVE_* macros are used

These are pre-existing newlib idioms — posix_spawn.c, popen.c, wordexp.c and the entire opendir family already wrap their bodies in #ifndef _NO_FOO / #ifndef HAVE_FOO. We're just turning the knobs the files were already prepared for.

The same pattern is used by other newlib targets (RTEMS, several embedded ports) — git grep _NO_POPEN newlib/configure.host for precedent.

Verification

Built with this branch on top of the pr-base-newlib-pr4 base branch (which carries the _NO_SIGPROCMASK / hash_page guard change this PR's diff depends on textually):

i686-nanvix-nm /opt/nanvix/i686-nanvix/lib/libc.a | grep -E ' T (regcomp|regexec|regerror|regfree|fnmatch|sleep|isatty|_isatty|creat)$'

shows all of them defined.

i686-nanvix-nm /opt/nanvix/i686-nanvix/lib/libc.a | grep -E ' U (sigprocmask|sigfillset|posix_spawn|popen|wordexp|getpwnam|issetugid|getlogin|_execve)$'

shows none — the silenced files contribute no new undefined references.

Follow-up work (tracked separately)

• Implementing real sigprocmask / posix_spawn / popen / wordexp / glob / execve on nanvix so these opt-outs can be removed.

Dependency

This PR depends textually on the _NO_SIGPROCMASK / hash_page change being applied first (both diffs modify the same newlib_cflags line in configure.host). The base of this PR is therefore set to pr-base-newlib-pr4, a temporary base branch built from dev + that prerequisite commit. When the prerequisite PR merges into dev, this PR can be rebased directly onto dev.

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com