seminaries/sum2vars/sum2vars-asm: add RISC-V variant

Signed-off-by: Pavel Pisa <pisa@cmp.felk.cvut.cz>

seminaries/sum2vars/sum2vars-asm/sum2vars-riscv.S

+.globl main
+.globl _start
+
+.text
+_start:
+main:
+	lw   x4, var_a(x0)
+	lw   x5, var_b(x0)
+	add  x6, x4, x5
+	sw   x6, var_c(x0)
+	
+	addi x2, x0, 0
+	jr   ra
+
+.data
+
+var_a:	.word 0x1234
+var_b:	.word 0x2222
+var_c:	.word 0x3333
+
+#pragma qtrvsim show registers
+#pragma qtrvsim show memory
+#pragma qtrvsim focus memory var_a
+#pragma qtrvsim tab core