[ARM] Replace branches to undefined weak functions with NOP

According to the code model (ARM, Thumb, Thumb2) this patch updates the b/bl/blx 0 instructions with NOP.
test/elf/ARM/weak-branch.test has been added with tests for all available NOP (A1, T1, T2 encodings).

Differential Revision: http://reviews.llvm.org/D8807

llvm-svn: 235498