target/i386: emulate 64-bit ring 0 for linux-user if LM feature is set

32-bit binaries can run on a long mode processor even if the kernel
is 64-bit, of course, and this can have slightly different behavior;
for example, SYSCALL is allowed on Intel processors.

Allow reporting LM to programs running under user mode emulation,
so that "-cpu" can be used with named CPU models even for qemu-i386
and even without disabling LM by hand.

Fortunately, most of the runtime code in QEMU has to depend on HF_LMA_MASK
or on HF_CS64_MASK (which is anyway false for qemu-i386's 32-bit code
segment) rather than TARGET_X86_64, therefore all that is needed is an
update of linux-user's ring 0 setup.

Fixes: https://gitlab.com/qemu-project/qemu/-/issues/1534


Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>