package/riscv64-elf-toolchain: new package

This commit adds a new package for a prebuilt bare-metal toolchain for
RISC-V 64-bit. Indeed, some bootloader/firmware for the BeagleV (and
potentially later for other platforms?) do not build with a
Linux-capable toolchain.

This uses a pre-built toolchain from SiFive, precompiled for x86-64,
so all packages using this toolchain must have the appropriate
BR2_HOSTARCH dependency.

This package is modeled after package/arm-gnu-a-toolchain/, which
package a pre-built ARM32 bare-metal toolchain.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>