target/i386: add support for FLUSH_L1D feature

As reported by Intel's doc:
"L1D_FLUSH: Writeback and invalidate the L1 data cache"

If this cpu feature is present in host, allow QEMU to choose whether to
show it to the guest too.
One disadvantage of not exposing it is that the guest will report
a non existing vulnerability in
/sys/devices/system/cpu/vulnerabilities/mmio_stale_data
because the mitigation is present only when the cpu has
	(FLUSH_L1D and MD_CLEAR) or FB_CLEAR
features enabled.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Message-Id: <20230201135759.555607-2-eesposit@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>