1.0.27.8: slightly faster x86oid pseudo atomic with {e,r}bp

1.0.27.8: slightly faster x86oid pseudo atomic with {e,r}bp

Trusting that within SBCL ebp is even and that it doesn't change
within a pseudo atomic section it's possible to use it to set and
clear pseudo atomic bits. It is ever so slightly faster (about 0.5%
overall on cl-bench on a P4).