This removes ice_unreachable() from the Subzero runtime.  The ice_unreachable() implementation is problematic since it makes a call to external function abort(), which leads to an undefined symbol error in the linker because abort() is internalized at link time.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4091
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/945953002