riscv: compat: s/failed/unsupported if compat mode isn't supported

riscv: compat: s/failed/unsupported if compat mode isn't supported

When compat mode isn't supported(I believe this is the most case now),
kernel will emit somthing as:
[    0.050407] riscv: ELF compat mode failed

This msg may make users think there's something wrong with the kernel
itself, replace "failed" with "unsupported" to make it clear. In fact
this is the real compat_mode_supported meaning. After the patch, the
msg would be:
[    0.050407] riscv: ELF compat mode unsupported

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Acked-by: Guo Ren <guoren@kernel.org>
Link: https://lore.kernel.org/r/20220821141819.3804-1-jszhang@kernel.org/
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>