kdump: Fix some cpp conditionals

[jhb]

Summary

While testing the bare values happens to work, the intent is to test
for their definition.

Fixes: 8a0f6d8cfdb7 ("kdump: Add preliminary support for decoding Linux syscalls")

[emaste]

I've been guilty of this myself. I wonder if we should make the first one #if defined(__amd64__) as well, for consistency