x/build: add LUCI linux-riscv64 builders with ISCAS #81534

ISCAS (Institute of Software, Chinese Academy of Sciences)
would like to request 3 types new LUCI builders for linux/riscv64.

ISCAS Lab has been a certified RISC-V Ecosystem Lab Partner since 2023
and has been actively contributing to the RISC-V toolchain and Go ecosystem.
The lab have three RISC-V machines available with different RVA profiles:

I propose the following hostname:

• linux-riscv64-rva20 linux-riscv64-rva20.csr.txt GORISCV64=rva20u64
• linux-riscv64-rva22 linux-riscv64-rva22.csr.txt GORISCV64=rva22u64
• linux-riscv64-rva23 linux-riscv64-rva23.csr.txt GORISCV64=rva23u64

I would like these builders to test Go on different RVA20/RVA22/RVA23 profiles like PPC (power9/10/11) does.
The existing rva20u64 builder will be gradually sunset @mengzhuo 's personal builders.

Please let me know if the hostname is acceptable or if I need to adjust the naming.

CC @golang/riscv64 @prattmic @dmitshur