[ELF] Introduce target specific inBranchRange() function

In preparation for range extension thunks introduce a function that will
check whether a branch identified by a relocation type at a source address
can reach a destination.

For targets where range extension thunks are not supported the function will
return true as it is not expected that branches are out of range. An
implementation has been provided for ARM.

Differential Revision: https://reviews.llvm.org/D34690

llvm-svn: 308188