arch: arm: cortex_a_r: Add cfi directives to z_arm_cortex_r_svc

Add call frame information (cfi) directives to the function
z_arm_cortex_r_svc, which triggers an SVC exception whose handler will
execute a context switch. The return address stored on the stack of a
suspended thread will point into z_arm_cortex_r_svc, that is, this
function will be the starting point when a debugger tries to produce a
call stack for a suspended thread. As the function is written in
assembly, most compilers will not emit any cfi automatically. This patch
adds cfi directives manually so that the call stack can be unwound.

The added cfi directives instruct the compiler to note in the
.debug_frame section that the canonical frame address is at offset 0
from r13, i.e., from the stack pointer.

Signed-off-by: Emil Kieri <emil.kieri@iar.com>

arch/arm/core/cortex_a_r/swap_helper.S

@@ -423,5 +423,9 @@ valid_syscall_id:
 
 GTEXT(z_arm_cortex_r_svc)
 SECTION_FUNC(TEXT, z_arm_cortex_r_svc)
+    .cfi_sections .debug_frame
+    .cfi_startproc simple
+    .cfi_def_cfa 13, 0x0
     svc #_SVC_CALL_CONTEXT_SWITCH
     bx lr
+    .cfi_endproc