Revert "Replaces vfork() implementation with fork()"

We're getting cold feet on this one... let's put it back.

This reverts commit 210331d9762037afb9b5ed8413079c6f65872df9.

Change-Id: I6b0d3c2b1dbf7f1dc9566979a91b7504c2189269
(cherry picked from commit 6a918870bab1a55a5f57dd7954abd9a8a27c1bc2)
diff --git a/libc/arch-mips/mips.mk b/libc/arch-mips/mips.mk
index bf3b8ae..8e415f9 100644
--- a/libc/arch-mips/mips.mk
+++ b/libc/arch-mips/mips.mk
@@ -59,6 +59,7 @@
     arch-mips/bionic/setjmp.S \
     arch-mips/bionic/sigsetjmp.S \
     arch-mips/bionic/syscall.S \
+    arch-mips/bionic/vfork.S \
 
 ifndef ARCH_MIPS_REV6
 libc_bionic_src_files_mips += \