linux(4): Deduplicate linux_fixup_elf().

Use native routines to fixup initial process stack. On Arm64 linux_elf_fixup() is
noop, as it do the stack fixup (room for argc) in the linux_copyout_strings().

MFC after: 1 week

(cherry picked from commit 9e550625f867a23ea3d87a77aa3c216b79ecd790)