arch: arm: update arch.mode in user thread system calls

This commit updates the thread.mode variable, in system
calls, whenever we modify the execution privilege level.
We need to do this, in order to be able to properly handle
context-switching triggered by ISRs, while doing the system
calls. The commit ensures that a context-switch, triggered
by an ISR during or after,a system call will preserve the
right privilege level.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>

arch/arm/core/swap_helper.S

@@ -418,6 +418,14 @@ _do_syscall:
     ldr r6, =K_SYSCALL_BAD
 
 valid_syscall_id:
+    push {r0, r1}
+    ldr r0, =_kernel
+    ldr r0, [r0, #_kernel_offset_to_current]
+    ldr r1, [r0, #_thread_offset_to_mode]
+    bic r1, #1
+    /* Store (privileged) mode in thread's mode state variable */
+    str r1, [r0, #_thread_offset_to_mode]
+    dsb
     /* set mode to privileged, r2 still contains value from CONTROL */
     bic r2, #1
     msr CONTROL, r2
@@ -427,6 +435,7 @@ valid_syscall_id:
      * instructions with the previous privilege.
      */
     isb
+    pop {r0, r1}
 
     /* return from SVC to the modified LR - _arm_do_syscall */
     bx lr