MIPS, SPARC: more fixes to the vfork aliases in libpthread.so

Commit 43c29487 tried to fix the vfork aliases in libpthread.so on MIPS
and SPARC, but failed to do it correctly, introducing an ABI change.

This patch does the remaining changes needed to align the MIPS and SPARC
vfork implementations with the other architectures. That way the the
alpha version of pt-vfork.S works correctly for MIPS and SPARC. The
changes for alpha were done in 82aab97c.

Changelog:
	* sysdeps/unix/sysv/linux/mips/vfork.S (__vfork): Rename into
	__libc_vfork.
	(__vfork) [IS_IN (libc)]: Remove alias.
	(__libc_vfork) [IS_IN (libc)]: Define as an alias.
	* sysdeps/unix/sysv/linux/sparc/sparc32/vfork.S: Likewise.
	* sysdeps/unix/sysv/linux/sparc/sparc64/vfork.S: Likewise.

(cherry picked from commit b87c1ec3)