bug in longlong.h for aarch64 sub_ddmmss

[Paul Zimmermann]

       Dear Torbjörn,

> This bug comes untimely for me.  I consider a major purge along the
> lines of the patch below. [...]

I confirm all MPFR 4.1.0-rc1 tests pass on gcc115 with this patch applied to
mpfr-longlong.h, both with GCC 9.1.0 and 10.1.0 (they failed with GCC 9.1.0),
where mpfr-longlong.h was copied from GMP 6.2.0, with minor changes.

If not already present, maybe some tests of the longlong.h routines that
use builtin_constant_p would be useful?

Paul