component PSA: keep SVCHandler_main/tfm_pendsv_do_schedule symbol in LTO builds

Add a "used" attribute to SVCHandler_main/tfm_pendsv_do_schedule to fix ARMC6 build with
the "-flto" flag.

This attribute, attached to a function/variable, means that code must be emitted
for the function even if it appears that the function is not referenced.

components/TARGET_PSA/TARGET_TFM/COMPONENT_SPE/secure_fw/core/ipc/tfm_spm.c

@@ -27,6 +27,7 @@
 #include "region_defs.h"
 #include "tfm_nspm.h"
 #include "tfm_memory_utils.h"
+#include "platform/mbed_toolchain.h"
 
 /*
  * IPC partitions.
@@ -580,7 +581,7 @@ void tfm_spm_init(void)
     tfm_thrd_start_scheduler(&this_thrd);
 }
 
-void tfm_pendsv_do_schedule(struct tfm_state_context_ext *ctxb)
+MBED_USED void tfm_pendsv_do_schedule(struct tfm_state_context_ext *ctxb)
 {
 #if TFM_LVL == 2
     struct spm_partition_desc_t *p_next_partition;

components/TARGET_PSA/TARGET_TFM/COMPONENT_SPE/secure_fw/core/tfm_handler.c

@@ -19,6 +19,7 @@
 #include <stdbool.h>
 #include "tfm_svcalls.h"
 #endif
+#include "platform/mbed_toolchain.h"
 
 /* This SVC handler is called when a secure partition requests access to a
  * buffer area
@@ -135,7 +136,7 @@ __attribute__((naked)) void SVC_Handler(void)
 #error "Unsupported ARM Architecture."
 #endif
 
-uint32_t SVCHandler_main(uint32_t *svc_args, uint32_t lr)
+MBED_USED uint32_t SVCHandler_main(uint32_t *svc_args, uint32_t lr)
 {
     uint8_t svc_number;
     /*