illumos gate

Review: https://www.illumos.org/rb/r/2505/

Testing Done

Nightly passed even without -Wno-unused-label for halt.

Performed disassembly diffs, nothing substantive changed:

nowhere(cmd/halt)[0]% dis halt > /tmp/after
nowhere(cmd/halt)[0]% wc -l /tmp/{before,after}
    4036 /tmp/before
    4036 /tmp/after
    8072 total
nowhere(cmd/halt)[0]% diff /tmp/{before,after}
1172c1172
<     get_zfs_bootfs_arg+0x2b5: 68 60 74 06 08     pushl  $0x8067460    <fmt.10798>
---
>     get_zfs_bootfs_arg+0x2b5: 68 60 74 06 08     pushl  $0x8067460    <fmt.10797>
nowhere(cmd/halt)[1]% grep unused *
nowhere(cmd/halt)[1]%