c/100547 - reject overly large vector_size attributes

c/100547 - reject overly large vector_size attributes

This rejects a number of vector components that does not fit an 'int'
which is an internal limitation of RTVEC.  This requires adjusting
gcc.dg/attr-vector_size.c which checks for much larger
supported vectors.  Note that the RTVEC limitation is a host specific
limitation (unless we change this 'int' to int32_t), but should be
32bits in practice everywhere.

2021-05-12  Richard Biener  <rguenther@suse.de>

	PR c/100547
gcc/c-family/
	* c-attribs.c (type_valid_for_vector_size): Reject too large nunits.
	Reword existing nunit diagnostic.

gcc/testsuite/
	* gcc.dg/pr100547.c: New testcase.
	* gcc.dg/attr-vector_size.c: Adjust.