tree-parloops: Enable runtime thread detection with -ftree-parallelize-loops

tree-parloops: Enable runtime thread detection with -ftree-parallelize-loops

This patch adds runtime thread count detection to auto-parallelization.
-ftree-parallelize-loops option generates parallelized loops without
specifying a fixed thread count, deferring this decision to program execution
time where it is controlled by the OMP_NUM_THREADS environment variable.

Bootstrap and regression tested on aarch64-linux.  Compiled SPEC HPC pot3d
https://www.spec.org/hpc2021/docs/benchmarks/628.pot3d_s.html with
-ftree-parallelize-loops and tested without having OMP_NUM_THREADS set in the
environment and with OMP_NUM_THREADS set to different values.

gcc/ChangeLog:

	* doc/invoke.texi (ftree-parallelize-loops): Update.
	* common.opt (ftree-parallelize-loops): Add alias that maps to
	special value INT_MAX for runtime thread detection.
	* tree-parloops.cc (create_parallel_loop): Use INT_MAX for runtime
	detection.  Call gimple_build_omp_parallel without building a
	OMP_CLAUSE_NUM_THREADS clause.
	(gen_parallel_loop): For auto-detection, use a conservative
	estimate of 2 threads.
	(parallelize_loops): Same.

gcc/testsuite/ChangeLog:

	* gcc.dg/autopar/runtime-auto.c: New test.

Signed-off-by: Sebastian Pop <spop@nvidia.com>