runtime: load sp and pc inside tinygo_longjmp

This is a small change to make it easier to support architectures that
need to restore more than just the sp and pc registers. In particular,
it is needed for the AVR architecture that needs to restore the frame
pointer (Y register).

src/runtime/asm_riscv.S

@@ -46,6 +46,7 @@ tinygo_scanCurrentStack:
 .global tinygo_longjmp
 tinygo_longjmp:
     // Note: the code we jump to assumes a0 is non-zero, which is already the
-    // case because that's jumpSP (the stack pointer).
-    mv sp, a0 // jumpSP
-    jr a1     // jumpPC
+    // case because that's the defer frame pointer.
+    lw sp, 0(a0)       // jumpSP
+    lw a1, REGSIZE(a0) // jumpPC
+    jr a1