Merge branch 'riscv64-sdi' into 'master'

am: add riscv64-sdi

See merge request projectn/nexus-am!41

am/arch/riscv64-sdi.mk

+include $(AM_HOME)/am/arch/isa/riscv64.mk
+include $(AM_HOME)/am/arch/platform/sdi.mk
+AM_SRCS += nemu/isa/riscv/boot/start.S

am/include/arch/riscv64-sdi.h

+#include "riscv64-nemu.h"

am/src/sdi/trm.c

@@ -25,7 +25,7 @@ void _halt(int code) {
   asm volatile (".byte 0xd6" : :"a"(code));
 #elif defined(__ISA_MIPS32__)
   asm volatile ("move $v0, %0; .word 0xf0000000" : :"r"(code));
-#elif defined(__ISA_RISCV32__)
+#elif defined(__ISA_RISCV32__) || defined(__ISA_RISCV64__)
   asm volatile("mv a0, %0; .word 0x0000006b" : :"r"(code));
 #else
 #error unsupport ISA