[LoongArch64] Fix nonvolatile context restoration referenced 101709. (#101904)

Author: shushanhf

src/coreclr/pal/src/arch/loongarch64/asmconstants.h

@@ -45,8 +45,8 @@
 #define CONTEXT_T6          CONTEXT_T5+SIZEOF_LOONGARCH_GPR
 #define CONTEXT_T7          CONTEXT_T6+SIZEOF_LOONGARCH_GPR
 #define CONTEXT_T8          CONTEXT_T7+SIZEOF_LOONGARCH_GPR
-#define CONTEXT_X0          CONTEXT_T8+SIZEOF_LOONGARCH_GPR
-#define CONTEXT_Fp          CONTEXT_X0+SIZEOF_LOONGARCH_GPR
+#define CONTEXT_R21         CONTEXT_T8+SIZEOF_LOONGARCH_GPR
+#define CONTEXT_Fp          CONTEXT_R21+SIZEOF_LOONGARCH_GPR
 #define CONTEXT_S0          CONTEXT_Fp+SIZEOF_LOONGARCH_GPR
 #define CONTEXT_S1          CONTEXT_S0+SIZEOF_LOONGARCH_GPR
 #define CONTEXT_S2          CONTEXT_S1+SIZEOF_LOONGARCH_GPR

src/coreclr/pal/src/arch/loongarch64/context2.S

@@ -132,13 +132,14 @@ LOCAL_LABEL(No_Restore_CONTEXT_INTEGER):
 
     ld.d  $ra, $t4, CONTEXT_Ra
     ld.d  $fp, $t4, CONTEXT_Fp
-    ld.d  $sp, $t4, CONTEXT_Sp
     ld.d  $r21, $t4, CONTEXT_Pc
-    ld.d  $t4, $t4, CONTEXT_T4
+    ld.d  $sp, $t4, CONTEXT_Sp
+    // NOTE: the r21 and t4 had been trashed.
     jirl  $r0, $r21, 0
 
 
 LOCAL_LABEL(No_Restore_CONTEXT_CONTROL):
+    ld.d  $r21, $t4, CONTEXT_R21
     ld.d  $t4, $t4, CONTEXT_T4
     jirl  $r0, $ra, 0
 LEAF_END RtlRestoreContext, _TEXT
@@ -202,7 +203,7 @@ LOCAL_LABEL(Done_CONTEXT_CONTROL):
     st.d  $t6, $a0, CONTEXT_T6
     st.d  $t7, $a0, CONTEXT_T7
     st.d  $t8, $a0, CONTEXT_T8
-    st.d  $r21,$a0, CONTEXT_X0
+    st.d  $r21,$a0, CONTEXT_R21
     st.d  $s0, $a0, CONTEXT_S0
     st.d  $s1, $a0, CONTEXT_S1
     st.d  $s2, $a0, CONTEXT_S2