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/SYSCALLS.TXT b/libc/SYSCALLS.TXT
index 128ec6b..38ae831 100644
--- a/libc/SYSCALLS.TXT
+++ b/libc/SYSCALLS.TXT
@@ -77,6 +77,7 @@
 int     setgroups:setgroups32(int, const gid_t*)   arm,x86
 int     setgroups:setgroups(int, const gid_t*)     arm64,mips,mips64,x86_64
 int     setpgid(pid_t, pid_t)  all
+pid_t   vfork(void)  arm
 int     setregid:setregid32(gid_t, gid_t)  arm,x86
 int     setregid:setregid(gid_t, gid_t)    arm64,mips,mips64,x86_64
 int     chroot(const char*)  all