[lld][ARM] support position independent thunks for Armv4(T)

- Position independent thunks now work for both Armv4 and Armv4T
- Armv4 arm->arm thunks don't emit a BX anymore, which doesn't exist for the
  arch. This fixes https://github.com/llvm/llvm-project/issues/50764.
- Armv4 and Armv4T both have the same arm->arm behaviour. Which also is
  desirable for the above ticket.

Reviewed By: MaskRay, peter.smith

Differential Revision: https://reviews.llvm.org/D141272