Add support on riscv64

### Is your feature request related to a problem?

Yes. The current grpc-java library supports the Linux-RISC-V64 architecture. This is a significant obstacle for Java developers who wish to build and deploy gRPC services and clients on RISC-V hardware. Without such support, users on RISC-V systems would not be able to use the large number of components related to this library, especially in the field of big data.

### Describe the solution you'd like
I want to add official support for the Linux-RISC-V64 architecture in grpc-java instead of using local patches for adaptation every time.

### Describe alternatives you've considered

Use local patches to enable grpc-java to adapt to the RISC-V64 architecture.

### Additional context
I have already ported and tested the necessary components (grpc-java) on real RISC-V hardware (e.g., SG2042, banana pi) and emulators (QEMU). The changes are minimal, primarily involving the addition of architecture detection and build flags for the new riscv64 target. However, the changes are also needed in `protoc-jar`.

And let me give my blogs and repos to solve these questions.

[links](https://gitee.com/wang-yijia_DB/spark-v3.5.1-on-riscv64)

Inizia esaminando il rilevamento delle architetture esistente e la gestione dei build flag in grpc-java, quindi analizza le modifiche correlate a protoc-jar menzionate nell’issue. Riproduci la configurazione segnalata su hardware Linux-RISC-V64 o in QEMU e confrontala con le architetture supportate. Il lavoro è completato quando la destinazione riscv64 è ufficialmente supportata senza patch locali e le build e i test pertinenti hanno esito positivo.

Scritto dal modello di indicizzazione a partire dal testo della issue.