Add BID decimal support

gcc/ChangeLog

gcc/Makefile.in

gcc/c-cppbuiltin.c

gcc/config.in

gcc/config/dfp-bit.c

gcc/config/dfp-bit.h

gcc/configure

gcc/configure.ac

gcc/dfp.c

gcc/doc/install.texi

gcc/optabs.c

gcc/testsuite/ChangeLog

gcc/testsuite/gcc.dg/dfp/convert-dfp.c

gcc/testsuite/gcc.dg/dfp/convert-int-saturate.c

gcc/testsuite/gcc.dg/dfp/decfloat-constants.c

libdecnumber/ChangeLog

libdecnumber/Makefile.in

libdecnumber/bid/bid-dpd.h

libdecnumber/bid/bid2dpd_dpd2bid.c

libdecnumber/bid/bid2dpd_dpd2bid.h