Add support to libkvm for reading vmcores from other architectures.

• Add a kvaddr_type to represent kernel virtual addresses instead of unsigned long.
• Add a struct kvm_nlist which is a stripped down version of struct nlist that uses kvaddr_t for n_value.
• Add a kvm_native() routine that returns true if an open kvm descriptor is for a native kernel and memory image.
• Add a kvm_open2() function similar to kvm_openfiles(). It drops the unused 'swapfile' argument and adds a new function pointer argument for a symbol resolving function. Native kernels still use _fdnlist() from libc to resolve symbols if a resolver function is not supplied, but cross kernels require a resolver.
• Add a kvm_nlist2() function similar to kvm_nlist() except that it uses struct kvm_nlist instead of struct nlist.
• Add a kvm_read2() function similar to kvm_read() except that it uses kvaddr_t instead of unsigned long for the kernel virtual address.
• Add a new kvm_arch switch of routines needed by a vmcore backend. Each backend is responsible for implementing kvm_read2() for a given vmcore format.
• Use libelf to read headers from ELF kernels and cores (except for powerpc cores).
• Add internal helper routines for the common page offset hash table used by the minidump backends.
• Port all of the existing kvm backends to implement a kvm_arch switch and to be cross-friendly by using private constants instead of ones that vary by platform (e.g. PAGE_SIZE). Static assertions are present when a given backend is compiled natively to ensure the private constants match the real ones.
• Enable all of the existing vmcore backends on all platforms. This means that libkvm on any platform should be able to perform KVA translation and read data from a vmcore of any platform.

Tested on: amd64, i386, sparc64 (marius)
Differential Revision: https://reviews.freebsd.org/D3341