Re-land r335297 "[X86] Implement more of x86-64 large and medium PIC code models"

Re-land r335297 "[X86] Implement more of x86-64 large and medium PIC code models"

Don't try to generate large PIC code for non-ELF targets. Neither COFF
nor MachO have relocations for large position independent code, and
users have been using "large PIC" code models to JIT 64-bit code for a
while now. With this change, if they are generating ELF code, their
JITed code will truly be PIC, but if they target MachO or COFF, it will
contain 64-bit immediates that directly reference external symbols. For
a JIT, that's perfectly fine.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@337740 91177308-0d34-0410-b5e6-96231b3b80d8