feat(userland): ported GNU binutils

[FlareCoding]

Note

Medium Risk
Adds a large third-party toolchain build and CI dependency; kernel syscall semantics for fcntl(F_GETFL) and new lstat can affect any userland that relies on those interfaces.

Overview
Adds a pinned GNU binutils 2.45 cross-build (make binutils) that installs static as, ld, objdump, and related tools into userland/toolchain/ for x86_64 and aarch64, using a new stellux-cc wrapper and optional patches. A binutils userland app copies those binaries into the initrd at /usr/bin, and CI caches userland/toolchain before kernel/userland builds. Setup docs, .gitignore, and toolchain-check are updated accordingly.

UEFI disk images (Makefile and flash scripts) grow from 256 MB to 512 MB to fit the larger rootfs.

Kernel fixes to support typical binutils/configure behavior: lstat on x86_64 (via newfstatat with AT_SYMLINK_NOFOLLOW), and fcntl(F_GETFL) now reports read/write access mode from handle rights instead of the handle flag word.

^{Reviewed by Cursor Bugbot for commit b2afd7d. Bugbot is set up for automated code reviews on this repo. Configure here.}