Various changes to get linker working again for a.out:

Various changes to get linker working again for a.out:
 * don't set/adjust section file positions or vmas more than once
 * use correct page size and segment size when calculating them
 * deal with some variations in a.out implementations
Tested on sun4 and sun4->sun3 so far, will be testing further but
needed to get wider exposure&testing.  See ChangeLog for details.

Also:
* coffcode.h (coff_write_relocs): Write out swapped reloc, not
pre-swapped version.
* hosts/sparc.h (abort, exit): Hide these names if compiling with
gcc version 2, to avoid warnings.