[ELF][ARM] Add getPCBias()

ThunkCreator::getThunk and ThunkCreator::normalizeExistingThunk
currently assume that the implicit addends are -8 for ARM and -4 for
Thumb. In D70637, ThunkCreator::getThunk will need to take care of the
relocation addend explicitly.

Add the utility function getPCBias() as a prerequisite so that the getThunk change in D70637
can be more general.

Reviewed By: peter.smith

Differential Revision: https://reviews.llvm.org/D70690

Change-Id: I6ecc45e88b14284c39bd3517a14c5277e93d7526
(cherry picked from commit 3d9b1128)