* defs.h: Provide prototypes for floatformat_is_negative,

* defs.h: Provide prototypes for floatformat_is_negative,
floatformat_is_nan and floatformat_mantissa.
* utils.c: Include "gdb_assert.h".
(floatformat_is_negative): New function.
(floatformat_is_nan): New function.
(floatformat_mantissa): New function.
* valprint.c: Include "floatformat.h".
(print_floating): Get rid of the Linux-specific
TARGET_ANALYZE_FLOATING macro and rewrite NaN detection with the
help these new functions.  Print NaN's in a format conforming to
ISO C99.