Use position-independent loads in asm.S

riscv-rt/asm.S

@@ -15,12 +15,10 @@ _start:
 
     // .option push
     // .option norelax
-    lui gp, %hi(__global_pointer$)
-    addi gp, gp, %lo(__global_pointer$)
+    la gp, __global_pointer$
     // .option pop
 
-    lui sp, %hi(_stack_start)
-    addi sp, sp, %lo(_stack_start)
+    la sp, _stack_start
 
     add s0, sp, zero