Merge pull request #259 from amitesh-singh/ami/fix_arch_compilation

fix compilation for arch linux

ch32v003fun/ch32v003fun.mk

-
-PREFIX?=riscv64-unknown-elf
+ifeq ($(OS),Windows_NT)
+    PREFIX?=riscv64-unknown-elf
+else
+    ifeq (, $(shell which riscv64-unknown-elf-gcc))
+        PREFIX?=riscv64-elf
+    else
+        PREFIX?=riscv64-unknown-elf
+    endif
+endif
 
 CH32V003FUN?=../../ch32v003fun
 MINICHLINK?=$(CH32V003FUN)/../minichlink