Add rtems_get_build_label()

Original author: sebastian.huber

Add the following directive:

/* Generated from spec:/rtems/config/if/get-build-label */

/**
 * @ingroup RTEMSAPIConfig
 *
 * @brief Gets the RTEMS build label.
 *
 * The build label is a user-provided string defined by the build
 * configuration.
 *
 * @return Returns the pointer to the RTEMS build label.
 *
 * @par Notes
 * The build label can be used to distinguish test suite results obtained from
 * different build configurations.  A use case is to record test results with
 * performance data to track performance regressions.  For this a database of
 * performance limits is required.  The build label and the target hash
 * obtained from rtems_get_target_hash() can be used as a key to obtain
 * performance limits.
 *
 * @par Constraints
 * @parblock
 * The following constraints apply to this directive:
 *
 * * The directive may be called from within any runtime context.
 *
 * * The directive will not cause the calling task to be preempted.
 * @endparblock
 */
const char *rtems_get_build_label( void );