Calculate automatically the padding necessary for lining up the

Calculate automatically the padding necessary for lining up the
iretq instruction used when Meltdown mitigation is effect.  It got
pushed off when an lfence was added in locore.S rev 1.107, resulting
in two signals being sent instead of one when iretq faulted, and
neither signal had the correct sigcontext info.  Update the makefile
rule for locore.o to verify that things are correct.

ok mlarkin@