kernel: support elf32 on x86_64.

* define ELF32_COMPAT to enable ELF32 macros.
* add a flag ELF_LOAD_USER_IMAGE_TEST_EXECUTABLE to only check the format.
It will be used by load_image_internal() to check which mode to use when
loading an image.
* in arch_elf_relocate_rel(), switch to elf_addr instead of addr_t, which
would be the wrong size for elf32 on x86_64.
* the ELF compat loader reuses the relevant parts of elf.cpp and arch_elf.cpp,
excluding for instance load_kernel_add_on() or dump functions.

Change-Id: Ifa47334e5adefd45405a823a3accbd12eee5b116