ports/qemu-riscv: Use -msave-restore to save code space

This is a RISC-V specific compiler switch which replaces explicit
push/pop register save/restore sequences with calls to functions which
do the same thing. The result is a nice savings of ROM space.

Signed-off-by: Keith Packard <keithp@keithp.com>

ports/qemu-riscv/Makefile

@@ -17,7 +17,7 @@ SNEK_ROOT = ../..
 SNEK_QEMU = $(SNEK_ROOT)/chips/qemu
 
 ARCH = riscv64-unknown-elf
-ARCH_CFLAGS = -march=rv32imac -mabi=ilp32
+ARCH_CFLAGS = -march=rv32imac -mabi=ilp32 -msave-restore
 PROGNAME = snek-qemu-riscv
 
 include $(SNEK_QEMU)/snek-qemu.defs