riscv: signal: fix signal_minsigstksz

riscv: signal: fix signal_minsigstksz

The init_rt_signal_env() funciton is called before the alternative patch
is applied, so using the alternative-related API to check the availability
of an extension within this function doesn't have the intended effect.
This patch reorders the init_rt_signal_env() and apply_boot_alternatives()
to get the correct signal_minsigstksz.

Fixes: e92f469b0771 ("riscv: signal: Report signal frame size to userspace via auxv")
Signed-off-by: Yong-Xuan Wang <yongxuan.wang@sifive.com>
Reviewed-by: Zong Li <zong.li@sifive.com>
Reviewed-by: Andy Chiu <andybnac@gmail.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20241220083926.19453-3-yongxuan.wang@sifive.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>