target/riscv: add rv32i, rv32e and rv64e CPUs

A bare bones 32 bit RVI CPU, rv32i, will make users lives easier when a
full customized 32 bit CPU is desired, and users won't need to disable
defaults by hand as they would with the rv32 CPU. [1] has an example of
a situation that would be avoided with rv32i.

In fact, add bare bones CPUs for RVE as well. Trying to use RVE in QEMU
requires one to disable every single default extension, including RVI,
and then add the desirable extension set. Adding rv32e/rv64e makes it
more pleasant to use embedded CPUs in QEMU.

[1] https://lore.kernel.org/qemu-riscv/258be47f-97be-4308-bed5-dc34ef7ff954@Spark/

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20240122123348.973288-3-dbarboza@ventanamicro.com>
[ Changes by AF:
 - Rebase on latest changes
]
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

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