[BOLT][NFC] Split createRelocation in X86 and share the second part

This commit splits the createRelocation function for the X86 architecture
into two parts, retaining the first half and moving the second half to a
new function called extractFixupExpr. The purpose of this change is to make
extractFixupExpr a shared function between AArch64 and X86 architectures,
increasing code reusability and maintainability.

Child revision: https://reviews.llvm.org/D156018

Reviewed By: Amir

Differential Revision: https://reviews.llvm.org/D157217