[elf-core] Improve reading memory from core file

This patch tries to improve memory-read from core files
(in order to improve disassembly functionality).

I am using RHEL 7.7 (linux kernel 3.10) and for a lot of cases,
I was not able to disassemble some functions from backtrace when
debugging crashes from core files. It outputs some dummy code.

The cause of the problem was the fact we are returning all the zeros
from ProcessElfCore::ReadMemory() that is being called within
Disassembler::ParseInstructions() and it disassembles some dummy
opcodes from the buffer returned. Therefore, we are removing zero
bytes filling (padding) completely.

Differential Revision: https://reviews.llvm.org/D93939