rt: fix some bugs for MIPS target

Author: crabtw

src/rt/arch/mips/_context.S

@@ -78,4 +78,5 @@ swap_registers:
         lw $31, 31 * 4($5)
 
         jr $31
+        nop
 .end swap_registers

src/rt/arch/mips/ccall.S

@@ -8,5 +8,22 @@
 __morestack:
         .set noreorder
         .set nomacro
+        move $7, $29
+        move $29, $6
 
+        sw $7, 0($29)
+        sw $31, -4($29)
+
+        addiu $29, $29, -24
+        move $25, $5
+        jalr $25
+        nop
+        addiu $29, $29, 24
+
+        lw $31, -4($29)
+        lw $7, 0($29)
+
+        move $29, $7
+        jr $31
+        nop
 .end __morestack

src/rt/arch/mips/gpr.h

@@ -21,10 +21,10 @@ class rust_gpr : public rust_gpr_base {
     uintptr_t r24, r25, r26, r27, r28, r29, r30, r31;
 
     inline uintptr_t get_fp() { return r30; }
-    inline uintptr_t get_ip() { return r0; }
+    inline uintptr_t get_ip() { return r31; }
 
     inline void set_fp(uintptr_t new_fp) { r30 = new_fp; }
-    inline void set_ip(uintptr_t new_ip) { r0 = new_ip; }
+    inline void set_ip(uintptr_t new_ip) { r31 = new_ip; }
 
     void load();
 };

src/rt/arch/mips/record_sp.S

@@ -14,6 +14,7 @@ record_sp_limit:
         addiu $3, $3, -0x7008
         sw $4, 4($3)
         jr $31
+        nop
 .end record_sp_limit
 
 .globl get_sp_limit
@@ -30,6 +31,7 @@ get_sp_limit:
         addiu $3, $3, -0x7008
         lw $2, 4($3)
         jr $31
+        nop
 .end get_sp_limit
 
 .globl get_sp
@@ -41,4 +43,5 @@ get_sp:
         .set nomacro
         move $2, $29
         jr $31
+        nop
 .end get_sp