Implement context switch for RV64

[ytakano]

Context switching for RV64:

https://github.com/tier4/awkernel/blob/main/awkernel_lib/src/context/rv64.rs

Examples:

• https://github.com/tier4/awkernel/blob/main/awkernel_lib/src/context/aarch64.rs
• https://github.com/tier4/awkernel/blob/main/awkernel_lib/src/context/x86_64.rs

Tests by using TLA+:

• https://github.com/tier4/awkernel/tree/main/specification/awkernel_lib/src/context/aarch64
• https://github.com/tier4/awkernel/tree/main/specification/awkernel_lib/src/context/x86