thread.c (call_without_gvl): spawn thread for UBF iff single-threaded

	We need another native thread to call some unblocking functions
	which aren't RUBY_UBF_IO or RUBY_UBF_PROCESS.  Instead of a
	permanent thread in <= 2.5, we can now rely on the thread cache
	feature to perform interrupts.

	[ruby-core:90865] [Bug #15499]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@66940 b2dd03c8-39d4-4d8f-98ff-823fe69b080e