[lld] Fix lld crash wrt generated thunks growing away from the patched code (PR #170495)

[Peter Smith via llvm-commits]

https://github.com/smithp35 commented:

Approach looks good to me. BTI Thunks didn't exist when the code to round up Thunk Size to 4 KiB was written, and I should have thought of the errata patches when adding them.

Most of my comments are comment and test suggestions. The one suggestion that isn't a simple change is whether we can avoid adding an additional symbol for the return value of the patch. The clang integrated assembler will convert a branch to local symbol to use section symbol + addend, we should be able to do the same.

https://github.com/llvm/llvm-project/pull/170495