The uses of MPFR in gimple-ssa-sprintf.c apparently cause
the library to allocates some internal caches that it then leaks
on program exit, causing Valgrind memory leak errors.  The MPFR
manual "strongly advises to [call mpfr_free_cache] before
terminating a thread, or before exiting when using tools like
'valgrind' (to avoid memory leaks being reported)) so the attached
patch does just that.

It' seems like an obvious fix that could presumably be committed
without a review or approval but I'd like to give others a chance
to comment on the placement of the call and whether it should be
guarded by ENABLE_VALGRIND_ANNOTATIONS.

Joseph, since you commented on the bug, do you have a suggestion
for a different site for it or a guard?  The only other call to
the function is in the Fortran FE and it's neither guarded nor
does it appear to ever be called.

Thanks
Martin