Add support for building iPXE as a 64-bit or 32-bit RISC-V binary, for
either UEFI or Linux userspace platforms.  For example:

  # RISC-V 64-bit UEFI
  make CROSS=riscv64-linux-gnu- bin-riscv64-efi/ipxe.efi

  # RISC-V 32-bit UEFI
  make CROSS=riscv64-linux-gnu- bin-riscv32-efi/ipxe.efi

  # RISC-V 64-bit Linux
  make CROSS=riscv64-linux-gnu- bin-riscv64-linux/tests.linux
  qemu-riscv64 -L /usr/riscv64-linux-gnu/sys-root \
               ./bin-riscv64-linux/tests.linux

  # RISC-V 32-bit Linux
  make CROSS=riscv64-linux-gnu- SYSROOT=/usr/riscv32-linux-gnu/sys-root \
       bin-riscv32-linux/tests.linux
  qemu-riscv32 -L /usr/riscv32-linux-gnu/sys-root \
               ./bin-riscv32-linux/tests.linux

Signed-off-by: Michael Brown <mcb30@ipxe.org>