target/i386: Assert !VM86 for x86_64 user-only

target/i386: Assert !VM86 for x86_64 user-only

For i386-linux-user, we can enter vm86 mode via the vm86(2) syscall.
That syscall explicitly returns to 32-bit mode, and the syscall does
not exist for a 64-bit x86_64 executable.

Since we're adding an accessor macro, pull the value directly out of
flags otherwise.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210514151342.384376-10-richard.henderson@linaro.org>