[Ada] The environ macro is broken on vxworks7r2 SR0610

[Ada] The environ macro is broken on vxworks7r2 SR0610

In SR0610, the TCB is made private, so the task environ field used by
the environ macro is no longer visible.  Arguably this is a bug, however
a more correct approach is to use accessor functions to retrieve this
field and not use the environ macro, thus avoiding the problem.

2019-07-10  Doug Rupp  <rupp@adacore.com>

gcc/ada/

	* env.c (__gnat_environ): Reformulate to also work for
	vxworks7r2 SR0610.

From-SVN: r273332