Clean up alloc overflow routine. Cross-compile needed.

src/bootfiles/20c/boot-2012-07-1-x86-cross.lisp:
o X86 cross-compile script

src/compiler/x86/macros.lisp:
o New inline-allocation to call our new overflow routine.

src/lisp/x86-assem.S:
o New alloc_overflow routine.

src/tools/cross-scripts/cross-x86-x86.lisp:
o Export all symbols in VM that are external symbols in OLD-VM with
  the corresponding symbol name.