riscv: fix offset in 64bit scheduler

Also keep common start.S file for 64 and 32 bit architectures.

src/runtime/scheduler_tinygoriscv64.S

@@ -3,30 +3,30 @@
 .type    tinygo_scanCurrentStack, %function
 tinygo_scanCurrentStack:
    // Push callee-saved registers onto the stack.
-   addi sp, sp, -128
-   sd ra,  120(sp)
-   sd s11, 112(sp)
-   sd s10, 104(sp)
-   sd s9,  96(sp)
-   sd s8,  88(sp)
-   sd s7,  80(sp)
-   sd s6,  72(sp)
-   sd s5,  64(sp)
-   sd s4,  56(sp)
-   sd s3,  48(sp)
-   sd s2,  40(sp)
-   sd s1,  32(sp)
-   sd s0,  24(sp)
+   addi sp, sp, -104
+   sd ra,  96(sp)
+   sd s11, 88(sp)
+   sd s10, 80(sp)
+   sd s9,  72(sp)
+   sd s8,  64(sp)
+   sd s7,  56(sp)
+   sd s6,  48(sp)
+   sd s5,  40(sp)
+   sd s4,  32(sp)
+   sd s3,  24(sp)
+   sd s2,  16(sp)
+   sd s1,  8(sp)
+   sd s0,  0(sp)
 
    // Scan the stack.
    mv a0, sp
    call tinygo_scanstack
 
    // Restore return address.
-   ld ra, 60(sp)
+   ld ra, 96(sp)
 
    // Restore stack state.
-   addi sp, sp, 128
+   addi sp, sp, 104
 
    // Return to the caller.
    ret