Update to GMP 6.2.0

[fingolfin]

This fixes a few bugs and brings performance improvements; for the full release notes, see https://gmplib.org/gmp6.2.

Removed deps/patches/gmp-config-ldflags.patch as it is no longer needed.

Adjusted deps/patches/gmp-exception.patch (in principle it could go, but then on Windows division by zero of GMP integers would not lead to a DivideError but instead a generic ErrorException)

This is the same as an improved version of #36309 which was reverted in #36328 as it was failing the pre-merge CI tests on Win32 and Win64. Moreover, there were other issues (see JuliaPackaging/Yggdrasil#1159 and JuliaRegistries/General#16517), so naturally this PR here shouldn't be merged hastily. But I wanted to submit it so that I can see what the failures on Win32 and Win64 are so that hopefully they can be debugged and resolved.

UPDATE: I added back a variant of the GMP exception pass to ensure behavior for division by zero of GMP bigints stayed the same on Windows, too. See also JuliaPackaging/Yggdrasil#1188

[fingolfin]

The Win32 precheck now works, but the main test fails -- I am pretty sure this is because JuliaPackaging/Yggdrasil#1188 isn't merged, so of course we are using the "old" GMP_jll 6.2 which had the old exceptions patch.

[PallHaraldsson]

@StefanKarpinski Should this get into 1.5 RC2 as it has bugfixes and "major speedup" (and supposedly tested already, as a separate project)? [And maybe your "active project" PR today as it's a small change (and at least safe for users; only for developers)?] [EDIT: I suppose it can be risky (I just thought 1.5 is a major update anyway), as you say it can go into 1.5.1, but not "active project" PR?]

[StefanKarpinski]

No, upgrading dependencies is quite risky and not something to do in a release candidate. It could go into 1.5.1 if it's unproblematic on master for a while.

[fingolfin]

This is indeed quite delicate. For now, GMP_jll 6.2.0 is set to require Julia >= 1.6 in the general registry; that would have to be adapted if it was put into Julia 1.5.1.

All in all, though, this might be a breaking change for other packages using GMP, so personally I would recommend to not put this into 1.5 but rather leave it for 1.6 (in the hope that 1.6 in turn won't be that far off ;-).