Update GMP to 6.3.0 to Avoid the x18 Register on Apple Silicon (#16686)

* Update GMP to 6.3.0 to Avoid the x18 Register on Apple Silicon

GMP 6.2.1's arm64 assembly keeps loop counters in x18, which Apple reserves and
macOS zeroes on every return to user space (syscalls, page faults, preemption).
When that happens inside one of those loops the counter underflows and the
routine walks the stack until the guard page: EXC_BAD_ACCESS / SIGBUS inside
__gmpn_* whenever CGAL falls back to exact arithmetic (texture import boundary
smoothing, text emboss cuts, mesh repair, mesh boolean).

6.3.0 moves those counters to ordinary temporaries (gmp-6.2 changeset
f4ff6ff711ed); no x18 use remains under mpn/arm64. The GCC 15 patch applies
unchanged apart from a line offset. The tarball comes from the GNU mirror, like
MPFR. Windows is unaffected: MSVC copies the prebuilt libgmp-10 and the arm64
job stages MSYS2's GMP.

Built with the deps recipe flags and ran make check on Linux x86_64 (clang 18,
200/200) and macOS arm64 (Apple clang, 199/199).

* Add gmplib.org as a Fallback Download for GMP

Same two-source pattern as MPFR: the GNU mirror first, the project's own
server second, one hash for both.

* Use the OrcaSlicer_deps Mirror as the GMP Fallback Download

SoftFever published gmp-6.3.0.tar.xz in SoftFever/OrcaSlicer_deps, the
same mirror 6.2.1 came from. Both sources serve the .tar.xz release file,
byte-identical (one SHA-256): the GNU server first, the mirror as fallback.

* Stage GMP 6.3.0 for the Flatpak Build

The Flatpak sandbox has no network, so the manifest pre-downloads every
dependency tarball into external-packages/. Point the GMP entry at the
6.3.0 .tar.xz the deps recipe now expects.

scripts/flatpak/com.orcaslicer.OrcaSlicer.yml

@@ -226,10 +226,10 @@ modules:
         sha256: f353e7b99bd0cbfc27ac9082de51acf32a8bc0b3e21ff9661ecca6f205ec1d81
         dest: external-packages/OpenVDB
 
-      # GMP 6.2.1
+      # GMP 6.3.0
       - type: file
-        url: https://github.com/SoftFever/OrcaSlicer_deps/releases/download/gmp-6.2.1/gmp-6.2.1.tar.bz2
-        sha256: eae9326beb4158c386e39a356818031bd28f3124cf915f8c5b1dc4c7a36b4d7c
+        url: https://ftp.gnu.org/gnu/gmp/gmp-6.3.0.tar.xz
+        sha256: a3c2b80201b89e68616f4ad30bc66aee4927c3ce50e33929ca819d5c43538898
         dest: external-packages/GMP
 
       # MPFR 4.2.2