[patchpoint] Add support for symbolic patchpoint targets to SelectionDAG and the

X86 backend.

The code generated for symbolic targets is identical to the code generated for
constant targets, except that a relocation is emitted to fix up the actual
target address at link-time. This allows IR and object files containing
patchpoints to be cached across JIT-invocations where the target address may
change.

llvm-svn: 235483