cortexm: fix stack size offset

The old code was broken and led to a HardFault in a rather convoluted
way:

 1. The CFA offset was incorrect, in fact it was not aligned (the stack
    is supposed to always be aligned to 4 bytes at least).
 2. This unaligned size was then used for stack size calculations.
 3. A stack that wasn't a multiple of 4 was allocated.
 4. The calleeSavedRegs struct (in `(internal/task.state).archInit`) was
    not correctly aligned.
 5. Writing to this struct resulted in a HardFault.

src/internal/task/task_stack_cortexm.S

@@ -114,9 +114,9 @@ tinygo_swapTask:
     pop {r4-r11, pc}
     #else
     pop {r4-r7}
-    .cfi_def_cfa_offset 5*9
+    .cfi_def_cfa_offset 5*4
     pop {r0-r3}
-    .cfi_def_cfa_offset 1*9
+    .cfi_def_cfa_offset 1*4
     mov r8, r0
     mov r9, r1
     mov r10, r2