x86/headers: 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.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Brian Gerst <brgerst@gmail.com>
Link: https://lore.kernel.org/r/20250310104256.123527-1-thuth@redhat.com