build/riscv: tell the build system about riscv64/head.S

build/riscv: tell the build system about riscv64/head.S

This allows to `make arch/riscv/riscv64/head.o`.

Example of rune on a fresh copy of the repository:
    make XEN_TARGET_ARCH=riscv64 CROSS_COMPILE=riscv64-linux-gnu- KBUILD_DEFCONFIG=tiny64_defconfig arch/riscv/riscv64/head.o

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Bob Eshleman <bobbyeshleman@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Acked-by: Connor Davis <connojdavis@gmail.com>