esp_shared_stack: enable shared stack function for riscv and reenable the unit test

components/riscv/expression_with_stack_riscv_asm.S

@@ -0,0 +1,43 @@
+// Copyright 2015-2020 Espressif Systems (Shanghai) PTE LTD
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+    .section .text
+
+    .global esp_shared_stack_invoke_function
+    .type esp_shared_stack_invoke_function, @function
+esp_shared_stack_invoke_function:
+    /* save current stack and return address */
+    mv t0, sp
+    mv t1, ra
+
+    /* Set shared stack as new stack pointer */
+    mv sp, a1
+    
+    /* store the ra and previous stack pointer in a safe place */
+    addi sp,sp,-4
+    sw  t0, 0(sp)
+    sw  t1, 4(sp)
+
+    /* call the subroutine */
+    jalr a0, 0
+
+    /* gets the ra and stack pointer saved previously */
+    lw  t0, 0(sp)    
+    lw  t1, 4(sp)
+    addi sp, sp, 4
+    
+    /* restore both ra and real stack pointer of current task */
+    mv ra, t1    
+    mv sp, t0
+    ret