[llvm-objdump] - Implement -z/--disassemble-zeroes.

This is https://bugs.llvm.org/show_bug.cgi?id=37151,

GNU objdump spec says that "Normally the disassembly output will skip blocks of zeroes.",
but currently, llvm-objdump prints them.

The patch implements the -z/--disassemble-zeroes option and switches the default to always
skip blocks of zeroes.

Differential revision: https://reviews.llvm.org/D56083

llvm-svn: 350823