openmp: Change omp_atv_default value and rename omp_atv_sequential to omp_atv_serialized.

openmp: Change omp_atv_default value and rename omp_atv_sequential to omp_atv_serialized.

While this is an OpenMP 5.1 change, it is undesirable to let people use different
values and then deal with ABI backwards compatibility in a year or two.

2020-07-09  Jakub Jelinek  <jakub@redhat.com>

	* omp.h.in (omp_alloctrait_value_t): Change omp_atv_default from
	2 to -1.  Add omp_atv_serialized and define omp_atv_sequential using
	it.  Remove __omp_alloctrait_value_max__.
	* allocator.c (omp_init_allocator): Handle omp_atv_default for
	omp_atk_alignment and omp_atk_pool_size.