RISC-V: Do no continue isa string parsing without correct XLEN

The isa string should begin with either rv64 or rv32. Otherwise, it is
an incorrect isa string. Currently, the string parsing continues even if
it doesnot begin with current XLEN.

Fix this by checking if it found "rv64" or "rv32" in the beginning.

Tested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>