[lldb] [Process/elf-core] Disable for FreeBSD vmcores

Recognize FreeBSD vmcores (kernel core dumps) through OS ABI = 0xFF
+ ELF version = 0, and do not process them via the elf-core plugin.
While these files use ELF as a container format, they contain raw memory
dump rather than proper VM segments and therefore are not usable
to the elf-core plugin.

Differential Revision: https://reviews.llvm.org/D114967