MIPS: emulate -ffunction-sections with macros in mmu-mips

Using a macro to put each function in its own .icode-based section
allows us to put the functions in IRAM _and_ have linker GC. This
removes a troublesome #ifdef BOOTLOADER_SPL on the X1000 target.

Change-Id: Ia7b59778f5c36b7970dee4280547e434a1f4fc5a

firmware/target/mips/ingenic_x1000/spl.lds

@@ -26,6 +26,7 @@ SECTIONS
     {
         *(.init.text);
         *(.text*);
+        *(.icode*);
     } > TCSM
 
     . = ALIGN(4);