illumos gate

Building on sparc with gcc 7:

    ld: guidance: removal of unused dependency recommended: libzfs.so.1
    ld: guidance: removal of unused dependency recommended: libgen.so.1
    ld: guidance: see ld(1) -z guidance for more information
    collect2: error: ld returned 1 exit status

However, gcc 4.4.4-il does not complain. Apparently the gcc 7 is smart enough to optimise out the case when fast_reboot is set to 2, therefore removing the need for libzfs and libgen and resulting the error above. However, with small update we can make the build process consistent across compilers.

We use parse_fastboot_args() with all platforms, and this will resolve the ld guidance issues above.