libgomp: Simplify 'parse_stacksize' call in 'libgomp/env.c:initialize_env'

libgomp: Simplify 'parse_stacksize' call in 'libgomp/env.c:initialize_env'

Minor fix-up for commit r13-2545-g9f2fca56593a2b87026b399d26adcdca90705685
"OpenMP, libgomp: Environment variable syntax extension".  Supplying an array
of three elements is misleading: 'parse_stacksize' only looks at 'params[0]'.
Just let the compiler figure out the array size (..., as done elsewhere, too).

	libgomp/
	* env.c (initialize_env): Simplify 'parse_stacksize' call.