[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