generic: groundwork for RISC-V

Add build infrastructure for RISC-V.

Signed-off-by: Zoltan HERPAI <wigyori@uid0.hu>
(cherry picked from commit 50c05f6cd7)

include/target.mk

@@ -260,6 +260,10 @@ ifeq ($(DUMP),1)
     CPU_CFLAGS_arc700 = -mcpu=arc700
     CPU_CFLAGS_archs = -mcpu=archs
   endif
+  ifeq ($(ARCH),riscv64)
+    CPU_TYPE ?= riscv64
+    CPU_CFLAGS_riscv64:=-mabi=lp64d -march=rv64imafdc
+  endif
   ifneq ($(CPU_TYPE),)
     ifndef CPU_CFLAGS_$(CPU_TYPE)
       $(warning CPU_TYPE "$(CPU_TYPE)" doesn't correspond to a known type)