add PIC relocations for x86_64, and use a simpler hack in place of x86_64_high_symbol()

add PIC relocations for x86_64, and use a simpler hack in place of x86_64_high_symbol()
This is Wolfgang Thaller's patch sent to cvs-ghc recently, with extra
commentary by me.  It turns out that this patch is not just a cleanup,
it is also necessary for GHCi to work on x86_64 with shared libraries,
because previously lookupSymbol() was creating jump-table entries for
all symbols looked up that resolved outside 2Gb, whereas Wolfgang's
version only generates jump-table entries for 32-bit symbol references
in object code that we load.