libdecnumber: Modify dconfig.h to Integrate with QEMU

libdecnumber: Modify dconfig.h to Integrate with QEMU

Modify the dconfig.h header file so that libdecnumber code integrates QEMU
configuration.   Specifically:

  - the WORDS_BIGENDIAN preprocessor macro is used in libdecnumber code to
    determines endianness.  It is derived from the existing QEMU macro
    HOST_WORDS_BIGENDIAN which is defined in config-host.h.

  - the DECPUN macro determines the number of decimal digits (aka declets) per
    unit (byte).  This is 3 for PowerPC DFP.

Signed-off-by: Tom Musta <tommusta@gmail.com>
Signed-off-by: Alexander Graf <agraf@suse.de>