Don't provide scalb/significand _FloatN aliases [BZ #31760]

scalb is a deprecated interface which was obsolescent in POSIX.1-2001,
removed in POSIX.1-2008, never made to C standard.  significant was
originally from BSD and never made in any standard.  Fix BZ #31760 by
not providing _FloatN aliases for them.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>