Core: Enable platform specific extensions to the core set of SVC handlers

Support is provided for both the PSA IPC and Library models.

To enable platform specific SVC handlers, add
-DPLATFORM_SVC_HANDLERS=True to the CMAKE command line.

When PLATFORM_SVC_HANDLERS is defined, user must provide an
implementation of:

   int32_t platform_svc_handlers(tfm_svc_number_t svc_num,
                                 uint32_t *ctx, uint32_t lr);

Change-Id: I5fc641038732d2630e954f6c9b12df929b65c24a
Signed-off-by: Alan DeMars <ademars@ti.com>