target/i386: do not set LM for 32-bit emulation "-cpu host/max"

32-bit targets by definition do not support long mode; therefore, the
bit must be masked in the features supported by the accelerator.

As a side effect, this avoids setting up the 0x80000008 CPUID leaf
for

   qemu-system-i386 -cpu host

which since commit 5a140b2 ("x86/cpu: Use max host physical address
if -cpu max option is applied") would have printed this error:

  qemu-system-i386: phys-bits should be between 32 and 36  (but is 48)

Reported-by: Nathan Chancellor <natechancellor@gmail.com>
Tested-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

target/i386/cpu.c

@@ -5073,6 +5073,11 @@ static uint64_t x86_cpu_get_supported_feature_word(FeatureWord w,
     } else {
         return ~0;
     }
+#ifndef TARGET_X86_64
+    if (w == FEAT_8000_0001_EDX) {
+        r &= ~CPUID_EXT2_LM;
+    }
+#endif
     if (migratable_only) {
         r &= x86_cpu_get_migratable_flags(w);
     }