libstdc++-v3 fix for mips64-linux-gnu

[Benjamin Kosnik]

>This patch matches a change that went into glibc a few days ago.  It
>prevents .mips2 from taking effect when compiling for ABIs that may
>have 64-bit addresses, since .mips2 would then load only 32-bit
>addresses from GOTs and other such lossages.  Ok to install?

yes. target-dependant bits like this can get checked in without waiting,
as you are the mips maintainer.

-benjamin