add tests for PWM

tests/Makefile

@@ -0,0 +1,17 @@
+GCC=riscv64-unknown-elf-gcc
+CFLAGS=-mcmodel=medany -std=gnu99 -O2 -fno-common -fno-builtin-printf
+LDFLAGS=-static -nostdlib -nostartfiles -lgcc
+
+default: pwm.riscv
+
+%.o: %.S
+	$(GCC) $(CFLAGS) -D__ASSEMBLY__=1 -c $< -o $@
+
+%.o: %.c
+	$(GCC) $(CFLAGS) -c $< -o $@
+
+%.riscv: %.o crt.o syscalls.o
+	$(GCC) -T link.ld $(LDFLAGS) $^ -o $@
+
+clean:
+	rm -f *.riscv *.o