[XRay] Change xray_instr_map sled addresses from absolute to PC relative for x86-64

xray_instr_map contains absolute addresses of sleds, which are relocated
by `R_*_RELATIVE` when linked in -pie or -shared mode.

By making these addresses relative to PC, we can avoid the dynamic
relocations and remove the SHF_WRITE flag from xray_instr_map.  We can
thus save VM pages containg xray_instr_map (because they are not
modified).

This patch changes x86-64 and bumps the sled version to 2. Subsequent
changes will change powerpc64le and AArch64.

Reviewed By: dberris, ianlevesque

Differential Revision: https://reviews.llvm.org/D78082