riscv: compat: process: Add UXL_32 support in start_thread

If the current task is in COMPAT mode, set SR_UXL_32 in status for
returning userspace. We need CONFIG _COMPAT to prevent compiling
errors with rv32 defconfig.
Signed-off-by: NGuo Ren <guoren@linux.alibaba.com>
Signed-off-by: NGuo Ren <guoren@kernel.org>
Tested-by: NHeiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20220405071314.3225832-14-guoren@kernel.orgSigned-off-by: NPalmer Dabbelt <palmer@rivosinc.com>

arch/riscv/kernel/process.c

@@ -98,6 +98,15 @@ void start_thread(struct pt_regs *regs, unsigned long pc,
 	}
 	regs->epc = pc;
 	regs->sp = sp;
+
+#ifdef CONFIG_64BIT
+	regs->status &= ~SR_UXL;
+
+	if (is_compat_task())
+		regs->status |= SR_UXL_32;
+	else
+		regs->status |= SR_UXL_64;
+#endif
 }
 
 void flush_thread(void)