[lldb] Add minidump save-core functionality to ELF object files

This change adds save-core functionality into the ObjectFileELF that enables
saving minidump of a stopped process. This change is mainly targeting Linux
running on x86_64 machines. Minidump should contain basic information needed
to examine state of threads, local variables and stack traces. Full support
for other platforms is not so far implemented. API tests are using LLDB's
MinidumpParser.

Reviewed By: clayborg

Differential Revision: https://reviews.llvm.org/D108233