Finally address this TODO in the assembler. This will help translate
non-IRT using programs (no ABI stability). The default scons testing
mode is non-IRT, so this helps with that. I haven't actually tested
this against scons yet, but I'm filling in the tests based on how
LLVM translates the same bitcode.

The filetype=asm is adjusted to omit the "*" and the "$".

The filetype=obj is adjusted to check for fixups with NullSymbols,
and also fill the assembler buffer at the instruction's immediate
field w/ the right constant.

The filetype=iasm is still TODO (hits an new assert in the Fixup's emit() function).

Reverts 7ad1bed9:
"Allow stubbing of called constant addresses using command line argument."
since this is now handled (except for iasm).

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4080
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1017373002