bdfp: truncdddf fixed

This patch fixes the comparison for overflow in truncdddf.c.
libdfp: Fixed the conversion from float to Decimal32 in x86.

Change extendsfsd in order to use _Decimal32 in the division of the
mantissa to fix the conversion in x86.
libdfp: Added more tests to the test-bfp-conversion.

Added tests for bfp conversion based on GCC's test suite.
For this, the patch also adds tests for extendxftd, extenddddf
and extendtdtf.