Ping: Add a prepare_pch_save target hook (fix many MIPS16 PCH failures)

[Richard Sandiford]

Ping for:

    http://gcc.gnu.org/ml/gcc-patches/2011-12/msg00273.html

To recap, this gets rid of some locally-cached state before writing
out the PCH file (which we have to do because some of that state is
not under GGC control).  The patch doesn't change the user's MIPS16
vs. non-MIPS16 setting for any code; that's decided by attributes on a
function-by-function basis, or by mips_base_mips16 in the global case.

Richard