PNaCl: Change ExpandVarArgs to work around invalid use of va_arg

I've encountered two uses of va_arg on an empty argument list: NaCl's
open() wrapper (now fixed), and 176.gcc in Spec2k.  Although this is
invalid use of va_arg (giving undefined behaviour), it's too awkward
to fix 176.gcc.

Work around this invalid usage by ensuring that the argument list
pointer points to a location on the stack rather than being
uninitialised.  va_arg will return undefined values as it does in the
usual native ABIs, rather than crashing.

We could add options for non-strict and strict modes (possibly with a
bounds check), but that's too much complication for now.

BUG=https://code.google.com/p/nativeclient/issues/detail?id=3338
TEST=test/Transforms/NaCl/*.ll + ran 176.gcc from Spec2k

Review URL: https://codereview.chromium.org/13510004

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