Multiple sections of a given type are combined, and any holes between

Multiple sections of a given type are combined, and any holes between
sections are filled with zeroes.

This changes the definition of `text', `data', and `bss' to use the
flags set by BFD, rather than requiring hardwired input section names
(for the normal case where one is using GNU ld and the elf2flt.ld linker
script, this should produce identical results, as that only ever
produces the three well-defined sections anyway).

Patch from Miles Bader <miles@gnu.org>.