[ELF] Test undefined weak symbol for Thumb narrow branch

Add a test for the R_ARM_THM_JUMP11 relocation to an undefined symbol. We
have to use yaml2obj as llvm-mc relaxes the narrow branch to a b.w which
uses the R_ARM_THM_JUMP24 relocation instead.
    
Differential Revision: https://reviews.llvm.org/D50234

llvm-svn: 338999