linux-user: added ELF coredump support for ARM target

linux-user: added ELF coredump support for ARM target

When target process is killed with signal (such signal that
should dump core) a coredump file is created.  This file is
similar than coredump generated by Linux (there are few execption
though).  Current version only supports ARM target coredumps
but it is easy to add support for other targets as it only needs
to define USE_ELF_COREDUMP and provide implementation for
elf_core_copy_regs() function.

See linux-user/elfload.c for more information.