frontend: Add novector C pragma

FORTRAN currently has a pragma NOVECTOR for indicating that vectorization should
not be applied to a particular loop.

ICC/ICX also has such a pragma for C and C++ called #pragma novector.

As part of this patch series I need a way to easily turn off vectorization of
particular loops, particularly for testsuite reasons.

This patch proposes a #pragma GCC novector that does the same for C
as gfortan does for FORTRAN and what ICX/ICX does for C.

I added only some basic tests here, but the next patch in the series uses this
in the testsuite in about ~800 tests.

gcc/c-family/ChangeLog:

	* c-pragma.h (enum pragma_kind): Add PRAGMA_NOVECTOR.
	* c-pragma.cc (init_pragma): Use it.

gcc/c/ChangeLog:

	* c-parser.cc (c_parser_while_statement, c_parser_do_statement,
	c_parser_for_statement, c_parser_statement_after_labels,
	c_parse_pragma_novector, c_parser_pragma): Wire through novector and
	default to false.

gcc/testsuite/ChangeLog:

	* gcc.dg/vect/vect-novector-pragma.c: New test.