ada: Always use -gnatg in run-time GPR files

ada: Always use -gnatg in run-time GPR files

This patch makes it so -gnatg is always passed to the compiler when
rebuilding the run-time library with the dedicated GPR files. Before
this patch, if a user rebuilt the run-time with -XADAFLAGS=XXX where
XXX didn't include "-gnatg", the build would immediately fail. This
case occurs when following the instructions in libada.gpr, which
use '-XADAFLAGS="-gnatn"'.

gcc/ada/

	* libgnat/libgnat_common.gpr: Unconditionally pass -gnatg.