i386: split smm helper (sysemu)

i386: split smm helper (sysemu)

smm is only really useful for sysemu, split in two modules
around the CONFIG_USER_ONLY, in order to remove the ifdef
and use the build system instead.

add cpu_abort() when detecting attempts to enter SMM mode via
SMI interrupt in user-mode, and assert that the cpu is not
in SMM mode while translating RSM instructions.

Signed-off-by: Claudio Fontana <cfontana@suse.de>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210322132800.7470-12-cfontana@suse.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>