Riscv64 support & glibc version overriding

[chanderlud]

• Adds a glibc-version option to build_tool which uses zigbuild to override the glibc version for precompiled binaries
• Adds a Rust target for riscv64 and adds detection of this architecture to build_tool
• Related: the example plugin workflow does not currently pass due to Flutter Android changes, this PR should be merged to resolve the issue

[knopp]

I'll need to migrate the Android build files first. I'll try to do it tomorrow.