Avoid tickling a linker bug from microblaze pt-vfork.S.

libpthread used to have its own vfork implementation that differed
from libc's only in having a pointless micro-optimization.  There is
no longer any use to having a separate copy in libpthread, but the
historical ABI requires a compatibility shim.  microblaze was trying
to be slightly too clever about how it did this, and tickled a linker
bug.  The linker bug should get fixed eventually, but there's no
reason for us to keep tickling it in the meantime.

This doesn't reuse the generic pt-vfork.c because microblaze doesn't
have IFUNC support yet, and it doesn't reuse aarch64/pt-vfork.c
because that fails to generate a tailcall (with GCC 7.1.1).

	* sysdeps/unix/sysv/linux/microblaze/pt-vfork.S: Don't include
	alpha/pt-vfork.S.  Provide own compat shim for vfork and __vfork.
	* sysdeps/unix/sysv/linux/microblaze/vfork.S: Add __libc_vfork alias.
	* sysdeps/unix/sysv/linux/microblaze/localplt.data:
	libpthread.so no longer references __errno_location.