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-x86/x86.mk b/libc/arch-x86/x86.mk
index 8aa2645..019dc8e 100644
--- a/libc/arch-x86/x86.mk
+++ b/libc/arch-x86/x86.mk
@@ -31,6 +31,7 @@
     arch-x86/bionic/__set_tls.c \
     arch-x86/bionic/sigsetjmp.S \
     arch-x86/bionic/syscall.S \
+    arch-x86/bionic/vfork.S \
 
 ## ARCH variant specific source files
 arch_variant_mk := $(LOCAL_PATH)/arch-x86/$(TARGET_ARCH_VARIANT)/$(TARGET_ARCH_VARIANT).mk