ATJ213x: minor changes to hwstub port

1) Make use of $gp addressing. This saves some bin size and makes
   code slightly faster.
2) Substitute jr with simple j instruction in exception handling.
   Code is small and j can easily encode target address.
3) Remove nop after eret in interrupt handler. According to mips32r2
   ISA manual eret does not have branch delay slot.

Change-Id: If63feb12eef189f08f7b50c832a8091be5e6f570

utils/hwstub/stub/atj213x/Makefile

@@ -8,7 +8,7 @@ AS=$(PREFIX)gcc
 OC=$(PREFIX)objcopy
 DEFINES=
 INCLUDES=-I$(CURDIR)
-GCCOPTS=-march=mips32r2 -G0 -msoft-float
+GCCOPTS=-Os -march=mips32r2 -G128 -msoft-float
 BUILD_DIR=$(CURDIR)/build/
 ROOT_DIR=$(CURDIR)/..
 

utils/hwstub/stub/atj213x/crt0.S

@@ -130,38 +130,34 @@ stack_munge_loop:
     bne t0, sp, stack_munge_loop
     addiu t0, 4
 
+    # setup Global Pointer
+    la gp, _gp
+
     # jump to C code with enabled interrupts
     la t0, main
     jr.hb t0
     ei
 
-    .extern exception_jmp_ctx_ptr
     .global tlb_refill_handler
     .section .exception.tlb_refill,"ax",%progbits
 
 tlb_refill_handler:
+    j restore_exception_jmp
     li a0, EXCEPTION_ADDR
-    la k0, restore_exception_jmp
-    jr k0
-    nop
 
     .global cache_error_handler
     .section .exception.cache_error,"ax",%progbits
 
 cache_error_handler:
+    j restore_exception_jmp
     li a0, EXCEPTION_ADDR
-    la k0, restore_exception_jmp
-    jr k0
-    nop
 
     .global general_exception_handler
     .section .exception.general_exception,"ax",%progbits
 
 general_exception_handler:
+    j restore_exception_jmp
     li a0, EXCEPTION_UNSP
-    la k0, restore_exception_jmp
-    jr k0
-    nop
 
 /* s0-s7 not saved as this are callee saved registers
  * CO_STATUS is not saved as nested interrupts are not supported
@@ -249,8 +245,6 @@ restore:
     addiu sp, sp, 84
     move sp, k1
     eret
-    nop
 
     .set reorder
     .set at
-

utils/hwstub/stub/atj213x/hwstub.lds

@@ -28,6 +28,11 @@ SECTIONS
             *(.rodata*)
 	    *(.data*)
             *(.rel.dyn)
+
+            . = ALIGN(8);
+            _gp = ABSOLUTE(. + 0x7ff0);
+
+            *(.sdata*)
 	} > IRAM
 
   .bss (NOLOAD) :