transaction_safe exceptions prevent libstdc++ building for some targets

[DJ Delorie]

Joe Seymour <joe.s@somniumtech.com> writes:
>> the msp430 -mlarge multilib failing to build with...
>>> configure: error: Unknown underlying type for size_t
>>> make[1]: *** [configure-target-libstdc++-v3] Error 1
>
> This is still reproducible.

FYI the underlying type is uint20_t

I think I've complained that libstdc++ has a hard-coded list, rather
than using a configure-time check, in the past...