[lld][ARM] support absolute thunks for Armv4T Thumb and interworking

changes:
- BLX: The Arm architecture versions that support the branch and link
  instruction (BLX), can rewrite BLs in place when a state change from Arm<->Thumb
  is required. Armv4T does not have BLX and so needs thunks for state changes.
- v4T Thumb long branches needed their own thunk. We could have used the v6M
  implementation, but v6M doesn't have Arm state and must resolve to rather
  inefficient stack reshuffling. We also can't reuse v7 thumb thunks as they use
  MOVV/MOVT, which wasn't available yet for v4T.
- Remove the `lack of BLX' warning. LLVM only supports Arm Architecture versions
  upwards of v4, which we now all support in LLD.
- renamed existing thunks to better reflect their use:
  ARMV5ABSLongThunk -> ARMV5LongLdrPcThunk,
  ARMV5PILongThunk -> ARMV4PILongThunk
- removed isCompatibleWith method from ARMV5ABSLongThunk and ARMV5PILongThunk,
  as they were identical to the ARMThunk parent class implementation.

Support for (efficient) position independent thunks for v4T will be added in a
follow-up patch, including possible related thunk renaming and code comment
cleanup.

Reviewed By: MaskRay, peter.smith

Differential Revision: https://reviews.llvm.org/D139888