openmp: Implement support for OMP_TARGET_OFFLOAD environment variable

This implements support for the OMP_TARGET_OFFLOAD environment variable
introduced in the OpenMP 5.0 standard, which controls how offloading
is handled.  It may be set to MANDATORY (abort if offloading cannot be
performed), DISABLED (no offloading to devices) or DEFAULT (offload to
device if possible, fall back to host if not).

2020-10-20  Kwok Cheung Yeung  <kcy@codesourcery.com>

	libgomp/
	* env.c (gomp_target_offload_var): New.
	(parse_target_offload): New.
	(handle_omp_display_env): Print value of OMP_TARGET_OFFLOAD.
	(initialize_env): Parse OMP_TARGET_OFFLOAD.
	* libgomp.h (gomp_target_offload_t): New.
	(gomp_target_offload_var): New.
	* libgomp.texi (OMP_TARGET_OFFLOAD): New section.
	* target.c (resolve_device): Generate error if device not found and
	offloading is mandatory.
	(gomp_target_fallback): Generate error if offloading is mandatory.
	(GOMP_target): Add argument in call to gomp_target_fallback.
	(GOMP_target_ext): Likewise.
	(gomp_target_data_fallback): Generate error if offloading is mandatory.
	(GOMP_target_data): Add argument in call to gomp_target_data_fallback.
	(GOMP_target_data_ext): Likewise.
	(gomp_target_task_fn): Add argument in call to gomp_target_fallback.
	(gomp_target_init): Return early if offloading is disabled.

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