Add the llvm2ice -sandbox option (false by default) to select between
native and sandboxed code generation.  Currently, it controls whether
the llvm.nacl.read.tp intrinsic is lowered to gs:[0x0] or a call to
__nacl_read_tp.

Change the asm output slightly for -ffunction-sections so that objdump
is more willing to provide a disassembly.

BUG= none
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/504963002