xtensa: 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.

This is a completely mechanical patch (done with a simple "sed -i"
statement).

Cc: Chris Zankel <chris@zankel.net>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20250314071013.1575167-39-thuth@redhat.com>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>