Fix make-syscalls.sh VDSO support for GCC 8.

Fix make-syscalls.sh VDSO support for GCC 8.

sysdeps/unix/make-syscalls.sh has support, used only by x32, for
generating IFUNCs for kernel VDSO symbols.  This support creates
IFUNCs by setting symbol types manually, which is bad for debug info
and does not work with current GCC mainline because it results in
errors from the checks on types of function aliases.

This patch fixes it to use the common __ifunc macro, which uses the
ifunc attribute when available and so works with GCC mainline.  Note
however that the original error resulted from an indirect inclusion of
a header declaring __gettimeofday from the generated sources, and
using __ifunc now relies on such an indirect inclusion remaining as it
means use of __typeof to determine the correct types.  If glibc's
headers change in such a way as to remove that indirect inclusion, it
will become necessary to change the syscalls.list syntax for VDSO
syscalls so the name of the header to include can be specified.

Tested (compilation only) with build-many-glibcs.py that this fixes
the build for x32 with GCC mainline.

	* sysdeps/unix/make-syscalls.sh: Use __ifunc to define symbols
	using VDSO.