target: mips: mark .startup.spl section as "ax"

this fixes "has non-ABS relocation R_MIPS_26 against symbol 'spl_main'"
error when linked with lld.

Change-Id: I6b584d8fdcae3f509f322b8557249bc2870904a3

firmware/target/mips/ingenic_jz47xx/crt0.S

@@ -44,7 +44,7 @@
     .extern main
     .global _start
 
-    .section .init.text
+    .section .init.text,"ax",%progbits
     .set    push
     .set    mips32
     .set    noreorder

firmware/target/mips/ingenic_x1000/spl-start.S

@@ -31,7 +31,7 @@
     .set noreorder
     .set noat
 
-    .section .startup.spl
+    .section .startup.spl, "ax", %progbits
 
 _spl_start:
     /* Clear data watchpoint */