MIPS, SPARC: fix wrong vfork aliases in libpthread.so

With recent binutils versions the GNU libc fails to build on at least
MISP and SPARC, with this kind of error:

  /home/aurel32/glibc/glibc-build/nptl/libpthread.so:(*IND*+0x0): multiple definition of `vfork@GLIBC_2.0'
  /home/aurel32/glibc/glibc-build/nptl/libpthread.so::(.text+0xee50): first defined here

It appears that on these architectures pt-vfork.S includes vfork.S
(through the alpha version of pt-vfork.S) and that the __vfork aliases
are not conditionalized on IS_IN (libc) like on other architectures.
Therefore the aliases are also wrongly included in libpthread.so.

Fix this by properly conditionalizing the aliases like on other
architectures.

Changelog:
	* sysdeps/unix/sysv/linux/mips/vfork.S (__vfork): Conditionalize
	hidden_def, weak_alias and strong_alias on [IS_IN (libc)].
	* sysdeps/unix/sysv/linux/sparc/sparc32/vfork.S: Likewise.
	* sysdeps/unix/sysv/linux/sparc/sparc64/vfork.S: Likewise.