Fix libnldbl_nonshared.a references to internal libm symbols (bug 23735).

The redirection of built-in functions such as sqrt in include/math.h
applies when the wrappers for those functions in libnldbl_nonshared.a
are built, resulting in references to internal names such as
__ieee754_sqrt that aren't actually exported from the shared libm.
(This applies for sqrt in 2.28, also for the round-to-integer
functions in current master because of my changes there.)  This patch
arranges for NO_MATH_REDIRECT to be used for all the affected
functions, and adds a test for those functions in
libnldbl_nonshared.a.

(We could of course choose to obsolete libnldbl_nonshared.a and
require that people building with -mlong-double-64 either include the
relevant headers and have a compiler supporting asm redirection, or
have some other means of achieving that redirection at compile time if
not including those headers.  But while we have libnldbl_nonshared.a,
it seems appropriate to fix such bugs in it.)

Tested for powerpc, and with build-many-glibcs.py.

	[BZ #23735]
	* sysdeps/ieee754/ldbl-opt/nldbl-compat.h (NO_MATH_REDIRECT):
	Define.
	* sysdeps/ieee754/ldbl-opt/test-nldbl-redirect.c: New file.
	* sysdeps/ieee754/ldbl-opt/Makefile [$(subdir) = math] (tests):
	Add test-nldbl-redirect.
	[$(subdir) = math] (CFLAGS-test-nldbl-redirect.c): New variable.
	[$(subdir) = math] ($(objpfx)test-nldbl-redirect): Depend on
	$(objpfx)libnldbl_nonshared.a.