arm64: get rid of fork/vfork/clone wrappers

[fixes from Catalin Marinas folded]

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Tested-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>