x86/traps: Initialize DR7 by writing its architectural reset value

Initialize DR7 by writing its architectural reset value to always set
bit 10, which is reserved to '1', when "clearing" DR7 so as not to
trigger unanticipated behavior if said bit is ever unreserved, e.g. as
a feature enabling flag with inverted polarity.

Signed-off-by: Xin Li (Intel) <xin@zytor.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Reviewed-by: Sohil Mehta <sohil.mehta@intel.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Sean Christopherson <seanjc@google.com>
Tested-by: Sohil Mehta <sohil.mehta@intel.com>
Cc:stable@vger.kernel.org
Link: https://lore.kernel.org/all/20250620231504.2676902-3-xin%40zytor.com