Spec2k now runs sandboxed, using all filetypes (asm, iasm, obj).

The new build-runtime.py script builds the native and sandboxed runtimes in build/runtime/ .  The various Subzero driver scripts are updated to use that.

Fixes a stack frame bug in sandboxed mode when the integrated assembler is used.  The stack adjustment for setting up a function call wasn't being rolled back for the second emitIAS() pass, so stack variables passed as arguments to the callee were being copied from the wrong stack slot.

Notes:

1. The hybrid Subzero/llc bisection debugging builds probably do not work as intended for -filetype=obj since the ELF emitter doesn't yet support -ffunction-sections.  (This was also true for non-sandboxed hybrid builds.)

2. The cross tests have not yet been adapted for testing sandboxing.  I'd prefer to first make progress on https://code.google.com/p/nativeclient/issues/detail?id=4085 in order to avoid blindly doubling the number of tests.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4079
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/944333002