math: Fix i386 and m68k fmod/fmodf on static build (BZ 31488)

The commit 16439f41 removed the static fmod/fmodf on i386 and m68k
with and empty w_fmod.c (required for the ABIs that uses the newly
implementation).  This patch fixes by adding the required symbols on
the arch-specific w_fmod{f}_compat.c implementation.

To statically build fmod fails on some ABI (alpha, s390, sparc) because
it does not export the ldexpf128, this is also fixed by this patch.

Checked on i686-linux-gnu and with a build for m68k-linux-gnu.

Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
Tested-by: Aurelien Jarno <aurelien@aurel32.net>