libcpp, libdecnumber: configure and substitute AR

AIX supports "FAT" libraries containing 32 bit and 64 bit objects
(similar to Darwin), but commands for manipulating libraries do not
default to accept both 32 bit and 64 bit object files.  While updating
the AIX configuration to support building and running GCC as a 64 bit
application, I have encountered some build libraries that hard code
AR=ar instead of testing the environment.

This patch adds AR_CHECK_TOOL(AR, ar) to configure.ac for the two
libraries and updates Makefile.in to accept the substitution.

2020-05-23  David Edelsohn  <dje.gcc@gmail.com>

libcpp/ChangeLog:
	* Makefile.in (AR): Substitute @AR@.
	* configure.ac (CHECK_PROG AR): New.
	* configure: Regenerate.

libdecnumber/ChangeLog:
	* Makefile.in (AR): Substitute @AR@.
	* configure.ac (CHECK_PROG AR): New.
	* configure: Regenerate.