PprMach.hs more accurate hack for x86-64, CmmLabelOff is not a relative reference.

PprMach.hs more accurate hack for x86-64, CmmLabelOff is not a relative reference.

CmmLabelDiffOff are generated in .text and if printed as .quad, causes
the assembler to emit a PC64 relocation. binutils prior to 2.17 don't
understand PC64 relocation properly.

pprDataItem is also used for printing SRT description tables. They are
part of the .data section and there no PC relative relocations are
emited. Hence, if we print a .long here, we get a absolute 32-bit
relocation. 32-bit relocations are problematic in dynamic libraries,
because dynamic library load addresses are loaded beyond the 32 bit
boundary, causing the dynamic linker to warn at dynamic linking
(loading the executable) that there are overflows in the
relocation. The executable still seems to work because of the small
memory model, but this is obviously wrong. Hence, remove CmmLabelOff
from the classification, causing these references to be printed as
.quad, causing correct 64-bit relocation as in the rest of the .data
section.

So, this hack now prints PC32 relocations in .text (mostly in the info
tables), and absolute 64-bit relocations in .data.