fixup "base: extend thread test for stack alignment"

@m-stein the fixup reveals that the main thread has an unaligned stack.
This results from x86_64/crt0.s which uses init_main_thread_result as
stack pointer and aftwards _calls_ into _main. But, init_main_thread_result
equals stack_top() which is since @cproc's patch no longer
_call_-compatible but _jmp_-compatible.