MIPS: Avoid a dangling `vfork@GLIBC_2.0' reference

MIPS: Avoid a dangling `vfork@GLIBC_2.0' reference

This satisfies a symbol reference created with:

	.symver	__libc_vfork, vfork@GLIBC_2.0

where `__libc_vfork' has not been defined or referenced.  In this case
the `vfork@GLIBC_2.0' reference is supposed to be discarded, however a
bug present in GAS since forever causes an undefined symbol table entry
to be created.  This in turn triggers a problem in the linker that can
manifest itself by link errors such as:

ld: libpthread.so: invalid string offset 2765592330 >= 5154 for section `.dynstr'

The GAS and linker bugs need to be resolved, but we can avoid them too
by providing a `__libc_vfork' definition just like our other platforms.

	[BZ #17485]
	* sysdeps/unix/sysv/linux/mips/vfork.S (__libc_vfork): Define.