Fix powerpc ifunc-sel.h build for -Os.

Compiling the testsuite for powerpc (multi-arch configurations) with
-Os with GCC 7 fails with:

In file included from ifuncmod1.c:7:0,
                 from ifuncdep1.c:3:
../sysdeps/powerpc/ifunc-sel.h: In function 'ifunc_sel':
../sysdeps/powerpc/ifunc-sel.h:12:3: error: asm operand 2 probably doesn't match constraints [-Werror]
   __asm__ ("mflr 12\n\t"
   ^~~~~~~
../sysdeps/powerpc/ifunc-sel.h:12:3: error: asm operand 3 probably doesn't match constraints [-Werror]
../sysdeps/powerpc/ifunc-sel.h:12:3: error: asm operand 4 probably doesn't match constraints [-Werror]
../sysdeps/powerpc/ifunc-sel.h:12:3: error: impossible constraint in 'asm'

The "i" constraints on function pointers require the function call to
be inlined so the compiler can see the constant function pointer
arguments passed to the asm.  This patch marks the relevant functions
as always_inline accordingly.

Tested that this fixes the -Os testsuite build for
powerpc-linux-gnu-power4, powerpc64-linux-gnu, powerpc64le-linux-gnu
with build-many-glibcs.py.

	* sysdeps/powerpc/ifunc-sel.h (ifunc_sel): Make always_inline.
	(ifunc_one): Likewise.