target/riscv: Update $ra with current $pc in trans_cm_jalt()

target/riscv: Update $ra with current $pc in trans_cm_jalt()

The original implementation sets $pc to the address read from the jump
vector table first and links $ra with the address of the next instruction
after the updated $pc. After jumping to the updated $pc and executing the
next ret instruction, the program jumps to $ra, which is in the same
function currently executing, which results in an infinite loop.
This commit stores the jump address in a temporary, updates $ra with the
current $pc, and copies the temporary to $pc.

Signed-off-by: Jason Chien <jason.chien@sifive.com>
Reviewed-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20240207081820.28559-1-jason.chien@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>