[nanvix] fix(z): sanitize in-source libc headers for C++ runtimes

[ppenna]

Problem

The #33 merge cleared the stage-1 configure stage; the build then failed compiling libunwind against the in-source Nanvix libc headers, which are C-only and don't parse as C++:

/opt/nanvix/usr/include/stdio.h:113: error: invalid parameter name: 'new' is a keyword
/opt/nanvix/usr/include/stdlib.h:92:  error: expected ')'        # the C99 'restrict' keyword
/opt/nanvix/usr/include/stdlib.h:163: error: invalid parameter name: 'template' is a keyword

libunwind, libc++ and libc++abi include these C headers from C++, so they must parse as C++.

Fix

Sanitize the staged sysroot copy of the headers right after staging (in z's download_steps):

• restrict (C99, not a C++ keyword) → __restrict (valid in both C and C++);
• drop C++-keyword parameter names (new, template) — parameter names in prototypes are optional, so this is purely syntactic.

Scope is tiny — restrict: 2 files; new: 1 decl; template: 4 decls — and harmless for compiler-rt's C consumers (both forms are valid in C).

Validation

• bash -n z clean.
• Verified with clang++ -fsyntax-only that the sanitized declarations parse cleanly, while the originals reproduce the exact CI errors.

Caveat

Iterative stage-1 bring-up: this clears the libunwind header blocker. The remaining libc++/libc++abi compile + link against the in-source libc may surface further issues, which I'll fix in follow-up iterations. Validated on merge (PR CI verifies stage 0 only).