This implements arm, armeb, thumb, thumbeb PLT entries parsing support
in ELF for llvm-objdump.
Implementation is similar to AArch64MCInstrAnalysis::findPltEntries. PLT
entry signatures are based on LLD code for PLT generation
(ARM::writePlt).
llvm-objdump tests are produced from lld/test/ELF/arm-plt-reloc.s,
lld/test/ELF/armv8-thumb-plt-reloc.s.
