ELF: Add support for short thunks on ARM.

A short thunk uses a direct branch (b or b.w) instruction, and is used
when the target has the same thumbness as the thunk and is within
direct branch range (32MB for ARM, 16MB for Thumb-2). Reduces the
size of Chromium for Android's .text section by around 160KB.

Differential Revision: https://reviews.llvm.org/D44963

llvm-svn: 328846