Format code to calculate thread stack and signal stack.

The code to calculate thread stack and signal stack looks weird:
the thread stack size and signal stack size are related with
each other on 32-bit mode, but not on 64-bit mode. So change the
code to make the logic more resonable. This doesn't change anything
as we have defined SIGSTKSZ to 16K on arm64.

Bug: 28005110

Change-Id: I04d2488cfb96ee7e2d894d062c66cef950fec418