[BZ #21745] powerpc: build some IFUNC math functions for libc and libm

Some math functions have to be distributed in libc because they're
required by printf.
libc and libm require their own builds of these functions, e.g. libc
functions have to call __stack_chk_fail_local in order to bypass the
PLT, while libm functions have to call __stack_chk_fail.

While math/Makefile treat the generic cases, i.e. s_isinff, the
multiarch Makefile has to treat its own files, i.e. s_isinff-ppc64.

	[BZ #21745]
	* sysdeps/powerpc/powerpc64/fpu/multiarch/Makefile:
	[$(subdir) = math] (sysdep_calls): New variable.  Has the
	previous contents of sysdep_routines, but re-sorted..
	[$(subdir) = math] (sysdep_routines): Re-use the contents from
	sysdep_calls.
	[$(subdir) = math] (libm-sysdep_routines): Remove the functions
	defined in sysdep_calls and replace by the respective m_* names.
	* sysdeps/powerpc/powerpc64/fpu/multiarch/s_isnan-ppc64.S:
	(compat_symbol): Undefine to avoid duplicated compat symbols in
	libc.

(cherry picked from commit 61c45f25)