[BOLT] Automatically detect and use relocations

Summary:
If relocations are available in the binary, use them by default.
If "-relocs" is specified, then require relocations for further
processing. Use "-relocs=0" to forcefully ignore relocations.

Instead of `opts::Relocs` use `BinaryContext::HasRelocations` to check
for the presence of the relocations.

(cherry picked from FBD6530023)