kernel: mark z_smp_current_get() with the const attribute

Repeated references to _current won't produce a different result as the
executing thread instance is always the same. Use the const attribute to
let the compiler know it may reuse a previously obtained value. This offset
the penalty for moving z_smp_current_get() out of line and provides yet
more binary size reduction.

This change is isolated in its own commit to ease bisecting in case some
unexpected misbehavior is eventually observed.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>

include/zephyr/kernel_structs.h

@@ -263,7 +263,7 @@ bool z_smp_cpu_mobile(void);
 #define _current_cpu ({ __ASSERT_NO_MSG(!z_smp_cpu_mobile()); \
 			arch_curr_cpu(); })
 
-struct k_thread *z_smp_current_get(void);
+__attribute_const__ struct k_thread *z_smp_current_get(void);
 #define _current z_smp_current_get()
 
 #else

kernel/smp.c

@@ -249,7 +249,7 @@ bool z_smp_cpu_mobile(void)
 	return !pinned;
 }
 
-struct k_thread *z_smp_current_get(void)
+__attribute_const__ struct k_thread *z_smp_current_get(void)
 {
 	/*
 	 * _current is a field read from _current_cpu, which can race