Ping: Fix MIPS va_arg regression

[Richard Sandiford]

Ping for:

    http://gcc.gnu.org/ml/gcc-patches/2012-01/msg01564.html

which fixes a MIPS va_arg regression (admittedly a long-standing one)
on zero-sized types.  There are no functional changes to other targets
and I'm as confident as I can be that it's safe for MIPS.

(It hasn't been a full week, but I'm nervous about missing the stage 4
cut-off.)

Thanks,
Richard