Implement an up to date and cross-platform <elf.h> for userland use.

With this change:

- Elftoolchain's <sys/elfdefinitions.h> is used for the definitions
that make up the ELF gABI and its processor-specific ABI
supplements.

- Existing NetBSD-local definitions are brought in via
the kernel header <sys/exec_elf.h>, for continuity.

This changeset prepares the ground for migrating userland-only
definitions out of the kernel header, for better separation
of concerns.

This changeset also eases the development of cross-architecture
ELF tools since <elf.h> now offers ELF-related constants for all
of the architectures that NetBSD currently supports.

PR lib/59564