meson.build: Merge riscv32 and riscv64 cpu family

In ba0e7333, we merged riscv32 and riscv64 in configure.
However, meson does not treat them the same.  We need to merge
them here as well.

Fixes: ba0e7333
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20211116095042.335224-1-richard.henderson@linaro.org
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>