package_linux: GCC 15.2 toolchain from GCCToolchain_jll

[maleadt]

This switches the toolchain of package_linux and package_linux_mmtk from GCC 9.1 (staticfloat/linux-gcc-toolchains, release tarballs without a recipe) to GCC 15.2 from the new GCCToolchain_jll (JuliaPackaging/Yggdrasil#14895, see there for the rationale):

• GCC 15.2.0 (now with gfortran) + binutils 2.45.1: the GCC that CompilerSupportLibraries_jll v1.5.7 is built with, with libstdc++ configured identically;
• unchanged glibc floor (2.17; 2.19 on aarch64/armv7l) and kernel headers (5.15.14).

GCC 9's headers are too old for clang to build LLVM, which is why the source (USE_BINARYBUILDER=0) and optimized builds of Julia run in llvm_passes and then require its glibc 2.36. With this, package_linux can serve those builds too, with the usual glibc floor.

Changes:

• install_gcc_toolchain pins the toolchain with a copy of the JLL's Artifacts.toml, and lets Pkg download and verify the artifact for the image's architecture (instead of piping unchecksummed tarballs into tar), then copies it into /usr/local. To bump, copy the Artifacts.toml of a newer JLL version (only together with CSL's GCC).
• The toolchain's runtime libraries (libstdc++ etc.) are copied over Debian's again, as Copy our GCC's libstdc++ over the system-wide libstdc++. #183 did before Upgrade default debian release from bullseye to bookworm #267: they are now newer than bookworm's, and programs built during a build must run.
• The sanity check now also compiles and runs C, C++ and Fortran programs and checks that they do not need a newer glibc than the floor.
• xz-utils and file, which source builds need and llvm_passes gets through build-essential.

Tested locally: install_gcc_toolchain (the code of this PR, with a stubbed sudo/chroot through bubblewrap) against copies of the v8.5 package_linux images of all five architectures (non-x86 through qemu-user), downloading from the JLL's release; and the artifact lookup on Julia 1.6. In the resulting x86_64 image, Julia's CI builds pass unchanged (x86_64-linux-gnu with -Werror, result needs at most GLIBC_2.17), and the source and optimized builds work too, given a few Julia build fixes that will come with moving them here.

Nothing uses the new images until julia-buildkite is pointed at the release that includes this.