Fix up bootstrap with GCC 4.[89] after RAII auto_mpfr and autp_mpz [PR109589]

Fix up bootstrap with GCC 4.[89] after RAII auto_mpfr and autp_mpz [PR109589]

On Tue, Apr 18, 2023 at 03:39:41PM +0200, Richard Biener via Gcc-patches wrote:
> The following adds two RAII classes, one for mpz_t and one for mpfr_t
> making object lifetime management easier.  Both formerly require
> explicit initialization with {mpz,mpfr}_init and release with
> {mpz,mpfr}_clear.

This unfortunately broke bootstrap when using GCC 4.8.x or 4.9.x as
it uses deleted friends which weren't supported until PR62101 fixed
them in 2014 for GCC 5.

The following patch adds an workaround, not deleting those friends
for those old versions.
While it means if people add those mp*_{init{,2},clear} calls on auto_mp*
objects they won't notice when doing non-bootstrap builds using
very old system compilers, people should be bootstrapping their changes
and it will be caught during bootstraps even when starting with those
old compilers, plus most people actually use much newer compilers
when developing.

2023-04-22  Jakub Jelinek  <jakub@redhat.com>

	PR bootstrap/109589
	* system.h (class auto_mpz): Workaround PR62101 bug in GCC 4.8 and 4.9.
	* realmpfr.h (class auto_mpfr): Likewise.