Fix bugs in (mostly) not-yet-activated parts of early/emergency output:

• map the hard-coded frame buffer address above KERNBASE. Using the physical address only worked because of larger mapping bugs.
  
  The hard-coded frame buffer address only works on x86. Use messy ifdefs to try to avoid warnings about unused code for other arches.

• remove the sysctl for reading and writing the table kernel console attributes. Writing only worked for emergency output since normal output uses unalterd copies.

• fix the test for the emergency console being usable

• explain why a hard-coded attribute is used very early. Emergency output works on x86 even before the pcpu pointer is initialized.