Partially revert r197863 to reduce diff against i386.

When I wrote the patch, I wanted to remove SYSINIT() usage from amd64 code.
There is no reason to keep the divergence any more because iwasaki merged
most amd64 suspend/resume code to i386 with r235622. Note this also fixed
an enge case reported by royger. [1]

Suggested by: jhb
Reviewed by: royger
Tested by: royger [1]
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D14400 [1]