libgomp/target.c: Fix buffer size for 'omp requires' diagnostic

libgomp/target.c: Fix buffer size for 'omp requires' diagnostic

One of the buffers that printed the list of set 'omp requires'
requirements missed the 'self' clause addition, being potentially
to short when all device-affecting clauses were passed. Solved it
by moving the sizeof(<string of all permitted values>" into a new
'#define' just above the associated gomp_requires_to_name function.

libgomp/ChangeLog:

	* target.c (GOMP_REQUIRES_NAME_BUF_LEN): Define.
	(GOMP_offload_register_ver, gomp_target_init): Use it for the
	char buffer size.