Fix i686 build with GCC 9.

Fix i686 build with GCC 9.

This patch fixes the glibc build for i686 with current mainline GCC,
where there are warnings about inconsistent attributes for aliases in
certain files defining libm IFUNCs.

In three of the files, the aliases were defined in terms of internal
symbols such as __sinf, and copied attributes from file-local
declarations of those functions which lacked the nothrow attribute.
Since the nothrow attribute is present on the declarations from
<math.h> (which include declarations of those __-prefixed functions),
the natural fix was to include <math.h> in those files, replacing the
local declarations.

In the other three files, a more complicated __hidden_ver1 call was
involved in the warnings.  <math.h> has not been included at this
point and, furthermore, it is included indirectly only later in the
source file after macros have been defined to remap a function name
therein.  So there isn't an obvious declaration from which to copy the
attribute and it seems simplest and safest just to add __THROW to the
hidden_ver1 calls.

Tested for i686 (build-many-glibcs.py compilers build for
x86_64-linux-gnu with GCC mainline; full testsuite run with GCC 7).

	* sysdeps/i386/i686/fpu/multiarch/e_expf.c [SHARED]: Use __THROW
	with __hidden_ver1 call.
	* sysdeps/i386/i686/fpu/multiarch/e_log2f.c [SHARED]: Likewise.
	* sysdeps/i386/i686/fpu/multiarch/e_logf.c [SHARED]: Likewise.
	* sysdeps/i386/i686/fpu/multiarch/s_cosf.c: Include <math.h>.
	(__cosf): Do not declare here.
	* sysdeps/i386/i686/fpu/multiarch/s_sincosf.c: Include <math.h>.
	(__sincosf): Do not declare here.
	* sysdeps/i386/i686/fpu/multiarch/s_sinf.c: Include <math.h>.
	(__sinf): Do not declare here.