target/i386: Add bus lock debug exception support

Bus lock debug exception is a feature that can notify the kernel by
generate an #DB trap after the instruction acquires a bus lock when
CPL>0. This allows the kernel to enforce user application throttling or
mitigations.

This feature is enumerated via CPUID.(EAX=7,ECX=0).ECX[bit 24].

Signed-off-by: Chenyi Qiang <chenyi.qiang@intel.com>
Message-Id: <20210202090224.13274-1-chenyi.qiang@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>