[PATCH] riscv: fix compitible with binutils 2.38

[Coelacanthus]

commit 6df2a016c0c8a3d0933ef33dd192ea6606b115e3 from linux kernel

Since binutils 2.38, default ISA spec version switch to 20191213,
in this version, original I extension be split into I, Zicsr and Zifencei.
Zicsr is csr read/write (csrr*/csrw*) instructions, and Zifencei
is fence.i instruction.
This will cause compile error like this:
    Error: unrecognized opcode `csrr a5,0xc01'
    Error: unrecognized opcode `fence.i'

This commit add code to detect new Zicsr and Zifencei extensions,
and enable it when needed.

Signed-off-by: Coelacanthus <coelacanthus at outlook.com>
Cc: Rick Chen <rick at andestech.com>
Cc: Leo <ycliang at andestech.com>
---
 arch/riscv/Makefile | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
index 0b80eb8d86..62712f8d38 100644
--- a/arch/riscv/Makefile
+++ b/arch/riscv/Makefile
@@ -24,7 +24,14 @@ ifeq ($(CONFIG_CMODEL_MEDANY),y)
 	CMODEL = medany
 endif
 
-ARCH_FLAGS = -march=$(ARCH_BASE)$(ARCH_A)$(ARCH_C) -mabi=$(ABI) \
+RISCV_MARCH_y = $(ARCH_BASE)$(ARCH_A)$(ARCH_C)
+
+# Newer binutils versions default to ISA spec version 20191213 which moves some
+# instructions from the I extension to the Zicsr and Zifencei extensions.
+toolchain-need-zicsr-zifencei := $(call cc-option-yn, -mabi=$(ABI) -march=$(RISCV_MARCH_y)_zicsr_zifencei)
+RISCV_MARCH_$(toolchain-need-zicsr-zifencei) := $(RISCV_MARCH_y)_zicsr_zifencei
+
+ARCH_FLAGS = -march=$(RISCV_MARCH_y) -mabi=$(ABI) \
 	     -mcmodel=$(CMODEL)
 
 PLATFORM_CPPFLAGS	+= $(ARCH_FLAGS)
-- 
2.36.1