xen/arm: Replace __ASSEMBLY__ with __ASSEMBLER__ in interface.h

xen/arm: Replace __ASSEMBLY__ with __ASSEMBLER__ in interface.h

While the GCC and Clang compilers already define __ASSEMBLER__
automatically when compiling assembly code, __ASSEMBLY__ is a
macro that only gets defined by the Makefiles in the kernel.
This can be very confusing when switching between userspace
and kernelspace coding, or when dealing with uapi headers that
rather should use __ASSEMBLER__ instead. So let's standardize now
on the __ASSEMBLER__ macro that is provided by the compilers.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Message-ID: <20260421142701.548978-1-thuth@redhat.com>