ARC: Replace __ASSEMBLY__ with __ASSEMBLER__ in uapi headers

__ASSEMBLY__ is only defined by the Makefile of the kernel, so
this is not really useful for uapi headers (unless the userspace
Makefile defines it, too). Let's switch to __ASSEMBLER__ which
gets set automatically by the compiler when compiling assembly
code.

Cc: linux-snps-arc@lists.infradead.org
Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Vineet Gupta <vgupta@kernel.org>