ARM: correct the relocation type for `bl` on WoA

The `IMAGE_REL_ARM_BRANCH20T` applies only to a `b.w` instruction.  A
thumb-2 `bl` should be relocated using a `IMAGE_REL_ARM_BRANCH24T`.
Correct the relocation that we emit in such a case.

Resolves PR38620!  Based on the patch by Jordan Rhee!

llvm-svn: 342109