feat: add support for aarch64 timer/interrupts, port MontaukOS userspace

toolchain/README.md

@@ -6,11 +6,16 @@ Two cross toolchains live here, both installed to `toolchain/local/`:
 |--------|--------|---------|
 | `build-toolchain.sh` | `x86_64-elf` | Bare-metal compiler for the kernel |
 | `build-montauk-toolchain.sh` | `x86_64-montauk` | OS-aware compiler for userspace programs |
+| `build-montauk-toolchain.sh aarch64` | `aarch64-montauk` | the same for the ARM port |
 
 Both use Binutils 2.43.1 + GCC 14.2.0 and share the source trees in
 `src/`. The Montauk target is added by the patches in `patches/` plus
 the target header `files/montauk.h` (installed as
-`gcc/config/montauk.h` in the GCC tree).
+`gcc/config/montauk.h` in the GCC tree). The AArch64 target is a second
+pair of patches (`*-montauk-aarch64.patch`) applied on top, with its
+target header `files/aarch64-montauk.h` (installed as
+`gcc/config/aarch64/montauk.h`). The ARM kernel is built with clang, not
+with a bare-metal GCC.
 
 ## The x86_64-montauk target
 
@@ -42,6 +47,27 @@ The in-tree programs are built `-fno-exceptions -fno-rtti` (the project
 convention), but that is a project style choice, not a toolchain limit —
 see the STL section below.
 
+## The aarch64-montauk target
+
+`./toolchain/build-montauk-toolchain.sh aarch64` builds
+`aarch64-montauk-gcc/g++` next to the x86 compilers, with its sysroot in
+`toolchain/sysroot-aarch64/` and its libc from
+`programs/lib/libc/build-aarch64/`. On a first build the libc cannot be
+compiled yet, so the script builds GCC against the headers alone and
+completes the sysroot afterwards. The target matches x86_64-montauk
+(static `ET_EXEC` at 0x400000, 4 KiB segment alignment, the same crt
+files and `-lc`), plus two defaults chosen so the shared userspace
+behaves the same on both architectures:
+
+- `-fsigned-char`: plain `char` is signed, as on x86 (AAPCS64 makes it
+  unsigned). The GUI's CP1252 text handling assumes signed bytes.
+- `-mno-outline-atomics`: inline LL/SC atomics. Outline atomics choose
+  LSE at run time from HWCAP, which Montauk does not provide, and their
+  helpers are in libgcc, which `-nostdlib` programs never link.
+
+`make ARCH=aarch64` at the repository root builds the userspace with it
+(into `programs/bin-aarch64/`); see `programs/mk/arch.mk`.
+
 ## C++ standard library (hosted libstdc++)
 
 `build-montauk-toolchain.sh` builds and installs a **hosted** libstdc++