Add GitHub Actions CI

The current approach is to build binutils/GCC/newlib from scratch. Among
the approaches that I tried, it seemed the easiest:

 - Downloading devkitPPC binaries: Tends to fail due to server-side rate-limiting
 - Building devkitPPC from source: Works, but lacks a little-endian libgcc
 - Enabling multilib in devkitPPC's GCC: GCC build fails for some reason
 - This approach: Takes about 10 minutes, but works

.github/build-binutils.sh

@@ -0,0 +1,19 @@
+#!/bin/sh
+# Build powerpc binutils (required for gcc)
+set -e
+
+VERSION=2.43.1
+MIRROR=https://ftpmirror.gnu.org/gnu
+DIR=binutils-$VERSION
+TARBALL=$DIR.tar.xz
+URL=$MIRROR/binutils/$TARBALL
+SHA512SUM=20977ad17729141a2c26d358628f44a0944b84dcfefdec2ba029c2d02f40dfc41cc91c0631044560d2bd6f9a51e1f15846b4b311befbe14f1239f14ff7d57824
+
+BASEDIR=$(pwd)
+wget -c $URL -O $TARBALL
+echo "$SHA512SUM $TARBALL" | sha512sum -c
+tar xf $TARBALL
+cd $DIR
+./configure --target=powerpc-eabi --prefix=$BASEDIR/toolchain --enable-poison-system-directories
+make
+make install

.github/build-gcc.sh

@@ -0,0 +1,20 @@
+#!/bin/sh
+# Build powerpc-none-eabi-gcc and libgcc for LE and BE.
+set -e
+
+VERSION=14.2.0
+MIRROR=https://ftpmirror.gnu.org/gnu
+DIR=gcc-$VERSION
+TARBALL=$DIR.tar.xz
+URL=$MIRROR/gcc/$DIR/$TARBALL
+SHA512SUM=932bdef0cda94bacedf452ab17f103c0cb511ff2cec55e9112fc0328cbf1d803b42595728ea7b200e0a057c03e85626f937012e49a7515bc5dd256b2bf4bc396
+
+BASEDIR=$(pwd)
+export PATH="$BASEDIR:$PATH"
+wget -c $URL -O $TARBALL
+echo "$SHA512SUM $TARBALL" | sha512sum -c
+tar xf $TARBALL
+mkdir -p $DIR/build; cd $DIR/build
+../configure --target=powerpc-eabi --enable-languages=c --disable-shared --disable-threads --disable-libssp --disable-analyzer --enable-poison-system-directories --with-newlib --with-headers=$BASEDIR/newlib-*/newlib/libc/include --prefix=$BASEDIR/toolchain
+make
+make install

.github/build-newlib.sh

@@ -0,0 +1,21 @@
+#!/bin/sh
+# Build powerpc newlib
+set -e
+
+VERSION=4.4.0.20231231
+MIRROR=https://sourceware.org/pub/
+DIR=newlib-$VERSION
+TARBALL=$DIR.tar.gz
+URL=$MIRROR/newlib/$TARBALL
+SHA512SUM=ea3baa0b7c9175aae024f0b7d272be092ef2c07483239a99329203e18a44bc23093d29e0ffcbe14bc591f610f0829eacd646cabb06d1c34aa23239cb1b814b46
+
+BASEDIR=$(pwd)
+wget -c $URL -O $TARBALL
+echo "$SHA512SUM $TARBALL" | sha512sum -c
+tar xf $TARBALL
+cd $DIR
+if [ "x$EXTRACT_ONLY" = x ]; then
+	./configure --host=powerpc-eabi --prefix=$BASEDIR/toolchain --disable-newlib-supplied-syscalls
+	make
+	make install
+fi

.github/workflows/build.yml

@@ -0,0 +1,51 @@
+name: Build maciNTosh
+
+on: [ push, pull_request ]
+
+jobs:
+  build:
+    runs-on: ubuntu-24.04
+
+    steps:
+    - uses: actions/checkout@v4
+
+    - name: Setup build
+      run: |
+        echo MAKEFLAGS="-j$(nproc || echo 1)" >> $GITHUB_ENV
+
+    - name: build binutils & GCC
+      run: |
+        sudo apt-get update && sudo apt-get install -y wget build-essential libmpfr-dev libgmp-dev libmpc-dev
+        .github/build-binutils.sh
+        EXTRACT_ONLY=1 .github/build-newlib.sh
+        .github/build-gcc.sh
+        .github/build-newlib.sh
+        make -C gcc-*/build install
+        echo "DEVKITPPC=$(pwd)/toolchain" >> $GITHUB_ENV
+
+    - name: build big endian libc
+      run: make -C baselibc
+
+    - name: build ARC firmware for Mac99 (unin)
+      run: |
+        mkdir -p arcunin/build
+        make -C arcloader_unin
+        make -C arcunin/baselibc
+        make -C arcunin
+
+    - name: build ARC firmware for Gossamer (grackle)
+      run: |
+        mkdir -p arcgrackle/build
+        make -C arcloader_grackle
+        make -C arcgrackle/baselibc
+        make -C arcgrackle
+
+    - uses: actions/upload-artifact@v4
+      with:
+        path: |
+          arcloader_*/stage1.elf
+          arcloader_*/stage1.elf.map
+          arc*/stage2.elf
+          arc*/stage2.elf.map
+
+    # TODO: create CD images