Tidy bits of libgcc/config/gthr-vxworks

Tidy bits of libgcc/config/gthr-vxworks

This addresses a variety of warnings about missing prototypes
or suspicious ptr-to-function conversions.

libgcc/
	* config/gthr-vxworks-thread.c (__init_gthread_tcb): Make static.
	(__delete_gthread_tcb): Likewise.
	(__task_wrapper): Likewise.
	(__gthread_create): Convert __task_wrapper to (void *) before going
	to (FUNCPTR).
	* config/gthr-vxworks-tls.c (tls_delete_hook): Accommodate prototype
	variations between kernel and rtp. Return STATUS.