0.7.3.10: Fix the SIGILL with ev6 and later Alphas: icache needs flushing

0.7.3.10: Fix the SIGILL with ev6 and later Alphas: icache needs flushing
after code objects are created e.g. by loading fasls

        * Move sanctify-for-execution from *-vm.lisp to
          compiler/target/sanctify.lisp, as *-vm is built too early to
          call VOPs

	* Add IMB to Alpha insts.lisp, and istream-memory-barrier VOP
          that calls it.

	* Create Alpha sanctify-for-execution function that calls said
	  new VOP

	* Also flush icache after GC

	* Double Alpha dynamic sizes to 256Mb: 128Mb is marginal to
	  rebuild SBCL

	* Minor Config.ppc-linux cleanup