Tests all cross tests in both sandboxed and unsandboxed modes.  Unfortunately, crosstest run time is more than doubled because of LTO of the crosstest drivers.  (We may want to add "full" and "lite" versions of cross tests.)

LLVM triple strings are removed where possible (from .ll files), and when generated, we use just i686 or i686-nacl.

"Fix" the integrated assembler to emit the lock prefix after the 16-bit operand prefix, to make the validator happy.

Don't add external symbol references to the ELF file for llvm.* intrinsic functions.

Make the ELF object writer honor the -externalize argument.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4092
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/973823003