This adds a Linux RV64 APE loader and teaches apelink to select it when uname -m reports riscv64. The loader uses the Linux generic syscall ABI and currently targets static RV64IMAC/LP64 ELF executables.

This is loader support only. It does not attempt to port Cosmopolitan libc or the compiler runtime to RISC-V (will be implemented in the future).

For a minimal test, save the following as /tmp/hello-riscv64.S:

	.section .text
	.globl _start
	.type _start, @function
_start:
	li a0, 1
	la a1, message
	li a2, 19
	li a7, 64
	ecall

	li a0, 0
	li a7, 93
	ecall
	.size _start, . - _start

	.section .rodata
message:
	.ascii "hello from riscv64\n"

It uses the Linux write and exit syscalls directly and does not depend on libc. Compile it with:

riscv64-linux-gnu-gcc \
  -nostdlib -static \
  -march=rv64imac -mabi=lp64 \
  -Wl,-e,_start \
  -Wl,--build-id=none \
  -Wl,-Ttext-segment=0x400000 \
  -o /tmp/hello-riscv64.elf \
  /tmp/hello-riscv64.S

The explicit text address keeps the ELF load segments above the 2 MiB userspace limit enforced by apelink.

Build apelink and the three loaders with:

make -j16 o//tool/build/apelink
make -j16 o//ape/ape.elf
make -j16 MODE=aarch64 o/aarch64/ape/ape.elf
make -j16 MODE=riscv64 ARCH=riscv64 \
  TOOLCHAIN=riscv64-linux-gnu- \
  o/riscv64/ape/ape.elf

The existing example ELFs can be used as the x86-64 and AArch64 payloads:

make -j16 o//examples/hello
make -j16 MODE=aarch64 o/aarch64/examples/hello

o//tool/build/apelink \
  -l o//ape/ape.elf \
  -l o/aarch64/ape/ape.elf \
  -l o/riscv64/ape/ape.elf \
  -o /tmp/hello.com \
  o//examples/hello.dbg \
  o/aarch64/examples/hello.dbg \
  /tmp/hello-riscv64.elf

Running the resulting file on x86-64 prints hello world. Running the same file on a Linux machine where uname -m reports riscv64 prints:

hello from riscv64

The RISC-V path was tested on real hardware, including repeated runs through the extracted loader cache. The existing x86-64 and AArch64 loader builds also continue to pass. (AArch64 execution was not tested on our hardware.)