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>

arch/Config.in

@@ -198,6 +198,17 @@ config BR2_powerpc64le
 	  http://www.power.org/
 	  http://en.wikipedia.org/wiki/Powerpc
 
+config BR2_riscv
+	bool "RISCV"
+	select BR2_ARCH_HAS_MMU_MANDATORY
+	select BR2_ARCH_NEEDS_GCC_AT_LEAST_7
+	help
+	  RISC-V is an open, free Instruction Set Architecture created
+	  by the UC Berkeley Architecture Research group and supported
+	  and promoted by RISC-V Foundation.
+	  https://riscv.org/
+	  https://en.wikipedia.org/wiki/RISC-V
+
 config BR2_sh
 	bool "SuperH"
 	select BR2_ARCH_HAS_MMU_OPTIONAL
@@ -423,6 +434,10 @@ if BR2_powerpc || BR2_powerpc64 || BR2_powerpc64le
 source "arch/Config.in.powerpc"
 endif
 
+if BR2_riscv
+source "arch/Config.in.riscv"
+endif
+
 if BR2_sh
 source "arch/Config.in.sh"
 endif