syscall emulation: fix fast build issue
syscall emulation: fix fast build issue

Surprisingly gcc will complain about unused variables even
inside an 'if (false)' block.

I thought I had tested this previously, but apparently not.
(transplanted from 563696c791d2944b223cefda2c1b09cee865ac32)

LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5