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).

deps/GMP/GMP.cmake

@@ -59,9 +59,12 @@ else ()
         set(_cross_compile_arg --host=${TOOLCHAIN_PREFIX})
     endif ()
 
+    # 6.3.0 is the first release whose arm64 assembly avoids x18, which macOS
+    # reserves and zeroes on every return to user space; 6.2.1 crashes on Apple
+    # silicon whenever CGAL falls back to exact arithmetic.
     ExternalProject_Add(dep_GMP
-        URL https://github.com/SoftFever/OrcaSlicer_deps/releases/download/gmp-6.2.1/gmp-6.2.1.tar.bz2
-        URL_HASH SHA256=eae9326beb4158c386e39a356818031bd28f3124cf915f8c5b1dc4c7a36b4d7c
+        URL https://ftp.gnu.org/gnu/gmp/gmp-6.3.0.tar.bz2
+        URL_HASH SHA256=ac28211a7cfb609bae2e2c8d6058d66c8fe96434f740cf6fe2e47b000d1c20cb
         DOWNLOAD_DIR ${DEP_DOWNLOAD_DIR}/GMP
         PATCH_COMMAND git apply ${GMP_DIRECTORY_FLAG} --verbose ${CMAKE_CURRENT_LIST_DIR}/0001-GMP_GCC15.patch
         BUILD_IN_SOURCE ON