we construct the EFI image. It doesn't seem to actually end up
in the EFI image, AFAICT.
o Replace .quad, .long and .short with data8, data4 and data2 resp.
The former are gnuisms.
o Redefine _start_plabel as a data16 with @iplt(_start) as its
value. This is the preferred way to create user PLT entries.