Signed vs unsigned lower halves of powerpc fixups are now handled by having two

Signed vs unsigned lower halves of powerpc fixups are now handled by having two
assembler directives, ha16() and has16(), for the upper half; has16() applies
the sign adjustment. .powerpcfixup is now gone, as we generate the relocation
in ha*() instead. Add special logic to the linker for undoing and redoing the
sign adjustment when reading/writing fixups. Tests still pass.