libgomp: Fix hang when profiling OpenACC programs with CUDA 9.0 nvprof

The version of nvprof in CUDA 9.0 causes a hang when used to profile an
OpenACC program.  This is because it calls acc_get_device_type from
a callback called during device initialization, which then attempts
to acquire acc_device_lock while it is already taken, resulting in
deadlock.  This works around the issue by returning acc_device_none
from acc_get_device_type without attempting to acquire the lock when
initialization has not completed yet.

2020-07-14  Tom de Vries  <tom@codesourcery.com>
	    Cesar Philippidis  <cesar@codesourcery.com>
	    Thomas Schwinge  <thomas@codesourcery.com>
	    Kwok Cheung Yeung  <kcy@codesourcery.com>

	libgomp/
	* oacc-init.c (acc_init_state_lock, acc_init_state, acc_init_thread):
	New variable.
	(acc_init_1): Set acc_init_thread to pthread_self ().  Set
	acc_init_state to initializing at the start, and to initialized at the
	end.
	(self_initializing_p): New function.
	(acc_get_device_type): Return acc_device_none if called by thread that
	is currently executing acc_init_1.
	* libgomp.texi (acc_get_device_type): Update documentation.
	(Implementation Status and Implementation-Defined Behavior): Likewise.
	* testsuite/libgomp.oacc-c-c++-common/acc_prof-init-2.c: New.

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