Rename OSCONS_CC to GCC_FOR_ADA_RTS

Rename OSCONS_CC to GCC_FOR_ADA_RTS

Several gnatlib* targets perform, with a subshell and sed, the same
GCC_FOR_TARGET pathname transformation that OSCONS_CC performs with
make subst macros.  Rename OSCONS_CC to a more general name, and use
it for gnatlib as well.

for  gcc/ada/ChangeLog

	* gcc-interface/Makefile.in (OSCONS_CC): Rename to...
	(GCC_FOR_ADA_RTS): ... this.  Adjust users.
	(gnatlib): Pass it down as CC.
	(gnatlib-shared-default): Likewise.
	(gnatlib-shared-win32, gnatlib-shared-darwin): Likewise.