Run r2 on the Game Boy Advance and boot Linux

[bokuweb]

Summary

Adds a gba/ frontend that runs the r2 RISC-V core on a Game Boy Advance and boots the bundled Linux (fixtures/linux.bin + default.dtb) to a shell.

• RAM: the GBA only has 288KB, so the upper 16MB of the 32MB GamePak window is used as guest RAM. This needs a flash cart that runs games out of PSRAM (or rgba with RGBA_PSRAM=1, Add flash-cart PSRAM emulation and a headless mode rgba#100). The kernel and DTB are embedded in the ROM and copied there at boot; the DTB memory size is patched the same way mini-rv32ima does.
• Speed: the interpreter loop lives in IWRAM and is built as ARM code (armv4t-none-eabi, ~2.2x faster than Thumb): ~70 ARM instructions per RISC-V instruction, ~110K guest IPS. The login prompt appears after ~6–7 minutes of GBA time.
• Clock: guest time comes from the instruction count (16 instructions/µs). At 1 instruction/µs the timer work starved userspace and the boot never reached the login prompt. WFI fast-forwards to the next timer interrupt.
• Console: 60x26 text on the Mode 3 bitmap (X11 misc-fixed 4x6 font, public domain), mirrored to the mGBA debug log. Login is answered automatically, uname -a runs at the first prompt, and each button types a command (A: uname -a, B: ls /, ...).

Core changes

• core and device_interfaces are now no_std; the Vec-backed Bus and start stay behind a default std feature, so app/wasm/wasi are unchanged.
• LR/SC reservation is a single Option<(addr, value)> (consumed by SC) instead of a HashMap.
• Cpu::bus_mut().

Notes

• Only tested on rgba. Real hardware also needs a PSRAM-capable flash cart and a fixed header (gbafix); the logo/checksum are not filled in.
• gba/ is its own workspace (nightly + build-std), so the root cargo check/CI does not build it.

Test plan

• cargo check, cargo test -p core
• make -C gba builds target/r2-gba.gba
• On rgba (RGBA_PSRAM=1): boots, logs in, uname -a → Linux buildroot 6.1.14 ... riscv32 GNU/Linux

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