m68k: Reorganize log1p and significand implementations

Commit 5e79e029 broke m68k after
s_significand.c became available in the build directory.  All m68k
implementations of log1p and significand were including s_significand.c
and stopped working after the inclusion of the the auto-generated file.

This patch reorganizes the implementation of log1p and significand for
m680x0 in order to avoid hitting this problem.

	* sysdeps/m68k/m680x0/fpu/s_log1p.c: Set as the generic file for
	all log1p and significand functions on m680x0.
	* sysdeps/m68k/m680x0/fpu/s_log1pf.c: Include s_log1p.c instead
	of s_significand.c..
	* sysdeps/m68k/m680x0/fpu/s_log1pl.c: Likewise.
	* sysdeps/m68k/m680x0/fpu/s_significandf.c: Likewise.
	* sysdeps/m68k/m680x0/fpu/s_significandl.c: Likewise.
	* sysdeps/m68k/m680x0/fpu/s_significand.c: Move all the code to
	s_log1p.c and include it..

Signed-off-by: Tulio Magno Quites Machado Filho <tuliom@linux.ibm.com>