swtch.s: During context save, use the correct bit mask for clearing

swtch.s: During context save, use the correct bit mask for clearing
         the non-reserved bits of dr7.

         During context restore, load dr7 in such a way as to not
         disturb reserved bits.

machdep.c: Don't explicitly disallow the setting of the reserved bits
           in dr7 since we now keep from setting them when we load dr7
           from the PCB.

           This allows one to write back the dr7 value obtained from
           the system without triggering an EINVAL (one of the
           reserved bits always seems to be set after taking a trace
           trap).

MFC after:	7 days