arm64: entry: Annotate ret_from_fork as code

In an effort to clarify and simplify the annotation of assembly
functions new macros have been introduced. These replace ENTRY and
ENDPROC with two different annotations for normal functions and those
with unusual calling conventions.

ret_from_fork is not a normal C function and should therefore be
annotated as code.

Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>