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

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@350823 91177308-0d34-0410-b5e6-96231b3b80d8