arm64: make asm/elf.h available to asm files

This reshuffles some code in asm/elf.h and puts a #ifndef __ASSEMBLY__
around its C definitions so that the CPP defines can be used in asm
source files as well.

Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>