extend.texi (Other Builtins): Add decimal float variants of signbit.

extend.texi (Other Builtins): Add decimal float variants of signbit.

	* doc/extend.texi (Other Builtins): Add decimal float variants
	of signbit.
	* builtins.def: Ditto.
	* builtins.c (expand_builtin): Ditto.
testsuite/
	* gcc.dg/dfp/signbit-1.c
	* gcc.dg/dfp/signbit-2.c
M    gcc/doc/extend.texi
M    gcc/builtins.c
M    gcc/ChangeLog
A    gcc/testsuite/gcc.dg/dfp/signbit-2.c
A    gcc/testsuite/gcc.dg/dfp/signbit-1.c
M    gcc/testsuite/ChangeLog
M    gcc/builtins.def

From-SVN: r123528