[Ada] Use VxWorks 7 APIs for accessing environment variables in kernel mode

[Pierre-Marie de Rodat]

VxWorks 7 provides newer APIs for kernel mode for accessing environment
variables that lead to smaller and more efficient code.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* env.c (__gnat_setenv): call setenv for VxWorks 7 kernel mode.
	(__gnat_environ): envGet takes an int instead of a NULL pointer.
	(__gnat_unsetenv): call unsetenv for VxWorks 7 kernel mode.
	(__gnat_clearenv): use __gnat_unsetenv to clear environment
	variables.
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