sourcebuild.texi: Document check_effective_target_newlib_nano_io.

sourcebuild.texi: Document check_effective_target_newlib_nano_io.

2018-11-26  Jozef Lawrynowicz  <jozef.l@mittosystems.com>

	gcc/ChangeLog:

	* doc/sourcebuild.texi: Document check_effective_target_newlib_nano_io.

	gcc/testsuite/ChangeLog:

	* lib/target-supports.exp (check_effective_target_newlib_nano_io): New.
	* gcc.c-torture/execute/920501-8.c: Register undefined linker symbol
	_printf_float for newlib_nano_io target.
	* gcc.c-torture/execute/930513-1.c: Likewise.
	* gcc.dg/torture/builtin-sprintf.c: Likewise.
	* gcc.c-torture/execute/ieee/920810-1.x: New.

From-SVN: r266488