core: zero bss and uibss; fix some section confusions

core: zero bss and uibss; fix some section confusions

Set .bss and .uibss as soon as we are fully loaded.  This gives us the
more familiar behavior of most normal execution environments.  The
.earlybss section is not zeroed; therefore, all variables that are set
before we have the opportunity to zero need to go in this section.

This checkin also fixes some incorrect section directives.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>