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_arm.S

@@ -40,6 +40,8 @@ tinygo_longjmp:
     .cfi_startproc
     // Note: the code we jump to assumes r0 is set to a non-zero value if we
     // jump from here (which is conveniently already the case).
+
+    ldm r0, {r0, r1}
     mov sp, r0 // jumpSP
     mov pc, r1 // jumpPC
     .cfi_endproc