arch: add support for RISC-V 64-bit (riscv64) architecture

arch: add support for RISC-V 64-bit (riscv64) architecture

This enables a riscv64 system to be built with a Buildroot generated
toolchain (gcc >= 7.x, binutils >= 2.30, glibc only).

This configuration has been used to successfully build a qemu-bootable
riscv-linux-4.15 kernel (https://github.com/riscv/riscv-linux.git).

Signed-off-by: Mark Corbin <mark.corbin@embecosm.com>
[Thomas:
 - simplify arch.mk.riscv by directly setting GCC_TARGET_ARCH
 - simplify glibc.mk changes by using GLIBC_CONF_ENV.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>