Create ELF core dump on coreclr exceptions and signals. (dotnet/coreclr#10902)

Create ELF core dump on coreclr exceptions and signals. (dotnet/coreclr#10902)

* Create ELF core dump on coreclr exceptions and signals.

When coreclr aborts because of a unhandled managed exception or hardware signal, the
"createdump" utility is launched if the "COMPlus_DbgEnableMiniDump" environment
variable is set to "1".

This utility uses the DAC enumerate memory interface to determine what memory regions
should be written for the managed state plus all the normal Linux memory regions for
the native state like threads, stacks, DSO, etc.

There still needs some to configure some of the "policies" like the name/location of
the core dump and the type of minidump (MiniDumpNormal, MiniDumpWithFullAuxiliaryState,
MiniDumpWithPrivateReadWriteMemory, etc.).

Currently only supported on Ubuntu Linux.

Make createdump smaller by using PAL in DAC module.

Commit migrated from https://github.com/dotnet/coreclr/commit/63cf53abae162743f1b217ee0ef3bbd3c505d097