https://github.com/openMSX/openMSX/commit/321d1d2f634d10ef72dc71831588540a7c6de59e

Index: build/detectsys.py
--- build/detectsys.py.orig
+++ build/detectsys.py
@@ -53,6 +53,8 @@ def detectCPU():
 		return 'sheb' if cpu.endswith('eb') else 'sh'
 	elif cpu == 'avr32':
 		return 'avr32'
+	elif cpu == 'riscv64':
+		return 'riscv64'
 	elif cpu == '':
 		# Python couldn't figure it out.
 		os = system().lower()