riscv64: support linux-gnu toolchain

am/arch/isa/riscv64.mk

-CROSS_COMPILE := riscv64-unknown-elf-
-COMMON_FLAGS  := -fno-pic -march=rv64im -mabi=lp64 -mcmodel=medany
+CROSS_COMPILE := riscv64-linux-gnu-
+COMMON_FLAGS  := -fno-pic -march=rv64imfd -mabi=lp64d -mcmodel=medany
 CFLAGS        += $(COMMON_FLAGS) -static
 ASFLAGS       += $(COMMON_FLAGS) -O0
 LDFLAGS       += -melf64lriscv

am/arch/riscv64-noop.mk

 include $(AM_HOME)/am/arch/isa/riscv64.mk
 
-CFLAGS += -march=rv64imc
+CFLAGS += -march=rv64imfd
 
 AM_SRCS := $(ISA)/noop/trm.c \
            $(ISA)/noop/uartlite.c \