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
diff --git a/libc/arch-mips64/mips64.mk b/libc/arch-mips64/mips64.mk
index 6f7a928..230cb26 100644
--- a/libc/arch-mips64/mips64.mk
+++ b/libc/arch-mips64/mips64.mk
@@ -46,6 +46,7 @@
     arch-mips64/bionic/setjmp.S \
     arch-mips64/bionic/sigsetjmp.S \
     arch-mips64/bionic/syscall.S \
+    arch-mips64/bionic/vfork.S \
 
 # FIXME TODO
 ## libc_bionic_src_files_mips64 += arch-mips64/string/memcpy.S