execve: Fix ABI change on unsized binaries

We need to adjust by the load page *after* we have checked for zero.
trs80: Check the PV flag not C on 8bit dec
ubee: check the right flag on dec b for the crtc loop.
execve: handle null files

When given an empty file execve was doing a bmap and trying to load block
-1 from the disk. Catch the empty file case earlier and error it.
crt0: set the load page to 1

Probably fcc binman should do the needed magic for such things