riscv_syscall.S: Remove duplicated code

Return from exception is common code for both system calls and
exceptions

arch/risc-v/src/common/riscv_exception_common.S

@@ -72,6 +72,7 @@
 
   .section EXCEPTION_SECTION
   .global exception_common
+  .global return_from_exception
   .align  8
 
 exception_common:
@@ -142,6 +143,8 @@ exception_common:
   addi       sp, sp, XCPTCONTEXT_SIZE
 #endif
 
+return_from_exception:
+
   /* If context switch is needed, return a new sp */
 
   mv         sp, a0

arch/risc-v/src/common/supervisor/riscv_syscall.S

@@ -119,52 +119,8 @@ sys_call6:
 
   /* Run the handler */
 
-  jal        x1, riscv_perform_syscall
-
-  /* Restore (potentially new) context */
-
-  mv         sp, a0                    /* use sp, as a0 gets wiped */
-
-  REGLOAD    s0, REG_EPC(sp)           /* restore epc */
-  csrw       CSR_EPC, s0
-
-  /* Restore status register, but don't enable interrupts yet */
-
-  REGLOAD    s0, REG_INT_CTX(sp)       /* restore status */
-  li         s1, STATUS_IE             /* move IE -> PIE */
-  and        s1, s0, s1                /* if (STATUS & IE) */
-  beqz       s1, 1f
-  li         s1, ~STATUS_IE            /* clear IE */
-  and        s0, s0, s1
-  li         s1, STATUS_PIE            /* set PIE */
-  or         s0, s0, s1
-
-1:
-  csrw       CSR_STATUS, s0
-
-#ifdef CONFIG_ARCH_KERNEL_STACK
-  /* Returning to userspace ? */
-
-  li         s1, STATUS_PPP
-  and        s0, s0, s1
-  bnez       s0, 1f
-
-  /* Set the next task's kernel stack to the scratch area */
-
-  jal        x1, riscv_current_ksp
-  csrr       s0, CSR_SCRATCH
-  REGSTORE   a0, RISCV_PERCPU_KSP(s0)
-
-1:
-#endif
-
-  load_ctx   sp
-
-  REGLOAD    sp, REG_SP(sp)            /* restore original sp */
-
-  /* return from exception, which updates the status register */
-
-  ERET
+  la         x1, return_from_exception
+  tail       riscv_perform_syscall
 
   .size sys_call0, .-sys_call0
   .size sys_call1, .-sys_call1