mips: Make the assembly-code N32-compatible.

There are no $t4 or $t5 in N32 but there are $a4 and $a5.

include/grub/mips/asm.h

@@ -0,0 +1,11 @@
+#ifndef GRUB_MIPS_ASM_HEADER
+#define GRUB_MIPS_ASM_HEADER	1
+
+#if defined(_MIPS_SIM) && defined(_ABIN32) && _MIPS_SIM == _ABIN32
+#define GRUB_ASM_T4 $a4
+#define GRUB_ASM_T5 $a5
+#else
+#define GRUB_ASM_T4 $t4
+#define GRUB_ASM_T5 $t5
+#endif
+#endif