[X86][NFC] Generalize the naming of "Retpoline Thunks" and related code to "Indirect Thunks"

There are applications for indirect call/branch thunks other than retpoline for Spectre v2, e.g.,

https://software.intel.com/security-software-guidance/software-guidance/load-value-injection

Therefore it makes sense to refactor X86RetpolineThunks as a more general capability.

Differential Revision: https://reviews.llvm.org/D76810