how to boot a mac into 64-bit mode by default

one can cause a mac to boot into 64 bit mode by holding down the 6 and the 4 keys during boot. Alternatively, one can edit the file /Library/Preferences/SystemConfiguration/com.apple.Boot.plist and add a kernel flag.

normally, the flag is an empty string:

        <key>Kernel Flags</key>
        <string></string>

change to:

        <key>Kernel Flags</key>
        <string>arch=x86_64</string>