[X86] Initialize TargetOptions::StackProtectorGuardOffset member to its default value

D88631 introduced a set of knobs to tweak how the stack protector is codegen'd for x86 targets, including the offset from the base register where the stack cookie is located. The `StackProtectorGuardOffset` field in `TargetOptions` was left uninitialized instead of being reset to its neutral value -1, making it possible to emit nonsensical code if the frontend doesn't change the field value at all before feeding the `TargetOptions` to the target machine initializer.

Reviewed By: nickdesaulniers

Differential Revision: https://reviews.llvm.org/D99952