Add RISC-V base instruction set multilibs

[stephanosio]

Add the multilibs for the following RISC-V base instruction sets:

• RV32I
• RV32E
• RV64I

The rationale is as follows:

• Base instruction set multilibs can be reused by (mapped to) every possible RISC-V extension combinations.
• Some FPGA-based platforms with a minimalistic soft RISC-V core may actually have a practical use of this.

Discussed in https://discord.com/channels/720317445772017664/733036686195294269/989508033543569448