Use longer jump in riscv/crt0.S

Simple `jal` fails to link when seL4 is built with user API
functions defined as public.

Signed-off-by: Marcin Witkowski <mwitkowski@antmicro.com>

crt/arch/riscv/crt0.S

diff --git a/crt/arch/riscv/crt0.S b/crt/arch/riscv/crt0.S
index e3354d2..a311d8b 100644
--- a/crt/arch/riscv/crt0.S
+++ b/crt/arch/riscv/crt0.S
@@ -32,10 +32,10 @@
 .option pop
 
 	li s0, 0
-	li ra, 0
 
 	addi  a0, sp, 0
-	jal  ra, __sel4_start_c
+	la x5, __sel4_start_c
+	jalr ra, x5, 0
 
 	/* should not return */
 1: